Characterization and manipulation of individual defects in insulating hexagonal boron nitride using scanning tunneling microscopy (1412.1878v1)

Abstract: Defects play a key role in determining the properties of most materials and, because they tend to be highly localized, characterizing them at the single-defect level is particularly important. Scanning tunneling microscopy (STM) has a history of imaging the electronic structure of individual point defects in conductors, semiconductors, and ultrathin films, but single-defect electronic characterization at the nanometer-scale remains an elusive goal for intrinsic bulk insulators. Here we report the characterization and manipulation of individual native defects in an intrinsic bulk hexagonal boron nitride (BN) insulator via STM. Normally, this would be impossible due to the lack of a conducting drain path for electrical current. We overcome this problem by employing a graphene/BN heterostructure, which exploits graphene's atomically thin nature to allow visualization of defect phenomena in the underlying bulk BN. We observe three different defect structures that we attribute to defects within the bulk insulating boron nitride. Using scanning tunneling spectroscopy (STS), we obtain charge and energy-level information for these BN defect structures. In addition to characterizing such defects, we find that it is also possible to manipulate them through voltage pulses applied to our STM tip.

• The paper introduces an STM method with a graphene capping layer to image and manipulate individual defect charge states in h-BN.
• Bright and dark dot observations reveal electron/hole asymmetry through variations in the local density of states.
• Voltage pulses are used to reversibly switch defect charge states, highlighting potential for atomically precise defect engineering.

Characterization and Manipulation of Defects in Hexagonal Boron Nitride via STM

Hexagonal boron nitride (h-BN) is a material with significant relevance in the field of two-dimensional materials, often used alongside graphene in various optoelectronic applications. Despite its utility, the properties and behavior of intrinsic defects within h-BN remain not fully explored, especially at the single-defect level within its insulating bulk. The paper reviewed here presents a novel approach for characterizing and manipulating individual defects in h-BN using scanning tunneling microscopy (STM), made feasible through a graphene/h-BN heterostructure.

Experimental Setup and Observations

The paper successfully utilizes the conductive nature of graphene to act as a capping layer over the insulating h-BN. By this method, the electronic interactions at the graphene/h-BN interface are probed using STM and scanning tunneling spectroscopy (STS), allowing defect imaging that would otherwise be untenable due to the lack of an inherent conductive path in h-BN. The researchers detail three primary defect structures: bright dots, dark dots, and ring features.

Bright and dark dots are inferred as positively and negatively charged defects within the bulk h-BN, respectively. This conclusion arises from differential conductance (dI/dV) trends in response to the proximity of the STM tip relative to defect centers. As the tip nears a bright dot, an escalation in local density of states (LDOS) above the Dirac point (V_s ≈ 0.1 V) is observed, whereas a reduction is noticed as it approaches dark dots. This electron/hole asymmetry indicates charged defects' capacity to perturb the electronic landscape of adjacent graphene.

Implications of Defect Manipulation

One of the innovative aspects of the paper is the manipulation of defect charge states through controlled voltage pulses. By applying a bias through the STM tip, the authors demonstrate reversible transitions between charged and neutral states, as well as polarity switches. This finding is pivotal as it suggests the capacity for defect engineering within h-BN, potentially influencing the local electronic properties of heterostructures through defect-mediated tuning.

Ring defects present an intriguing case; their radius changes in response to the applied gate voltage (V_g) and serve as indicators of charge transfer between defect states and graphene. This insight provides a mechanism to quantify defect energy levels, situating some defect levels approximately 30 meV above the graphene Dirac point. The evidence suggests a correlation with carbon impurities, which have been theoretically regarded as potential dopants in h-BN.

Theoretical Interpretations and Future Directions

From a theoretical perspective, the researchers back their observations with insights from prior electron paramagnetic resonance and luminescence studies, recognizing nitrogen vacancies and carbon impurities as possible defect types contributing to the observed phenomena. The systematic use of a graphene capping layer to enable insight into insulating bulk materials opens avenues for exploration in other similar material systems, such as diamond featuring nitrogen-vacancy centers.

Conclusion

This paper extends the boundaries of atomic-scale defect characterization in insulating substrates by leveraging conductive overlays like monolayer graphene. As the demand for precision in electronic material engineering escalates, methodologies such as described herein are likely to become foundational tools in developing advanced materials. Advanced manipulation techniques revealing atomically precise charge states reassert the relevance of h-BN in the field of nanoelectronic devices, thereby opening up fresh opportunities for the integration of h-BN in innovative technological designs.