Gate-Controlled Ionization and Screening of Cobalt Adatoms on a Graphene Surface

Abstract: We describe scanning tunneling spectroscopy (STS) measurements performed on individual cobalt (Co) atoms deposited onto backgated graphene devices. We find that Co adatoms on graphene can be ionized by either the application of a global backgate voltage or by the application of a local electric field from a scanning tunneling microscope (STM) tip. Large screening clouds are observed to form around Co adatoms ionized in this way, and we observe that some intrinsic graphene defects display a similar behavior. Our results provide new insight into charged impurity scattering in graphene, as well as the possibility of using graphene devices as chemical sensors.

• The paper demonstrates reversible, gate-induced ionization of cobalt adatoms, forming large screening clouds observable via STS.
• The study employs an ultra-high vacuum LT-STM on graphene prepared by exfoliation or CVD to achieve precise local measurements.
• Key spectroscopy features—including a Fermi-level dip, resonant peaks, and energy-reversing signals—reveal detailed charge impurity dynamics.

Gate-Controlled Ionization and Screening of Cobalt Adatoms on a Graphene Surface

The paper investigates the relationship between cobalt (Co) adatoms and graphene surfaces, focusing on their electronic interactions and the potential implications for graphene-based device applications. The study employs scanning tunneling spectroscopy (STS) to perform a detailed local-probe exploration of Co adatoms on graphene, contrasting spatially averaged techniques such as transport measurements and Raman spectroscopy, which typically obscure microscopic details.

Methodology

The researchers utilized an Omicron LT-STM in ultra-high vacuum conditions to study single Co adatoms on graphene sheets, prepared either through mechanical exfoliation or chemical vapor deposition. These graphene samples were equipped with a backgate to modulate charge carrier density electrically. Co adatoms, introduced via e-beam evaporation, were found to ionize under global backgate voltage or local electric fields from the STM tip, forming distinguishable screening clouds.

Key Findings

The study presents several novel findings regarding Co adatoms on graphene:

1. Ionization and Screening: Co adatoms can be reversibly ionized via gate voltage modulation, forming large screening clouds that are observable with STS. This behavior was analogous to the response seen over certain intrinsic graphene defects.
2. Spectroscopy Features: STS measurements revealed three distinct types of features associated with Co adatoms:
   • A dip at the Fermi level (E_F) was speculated to result from either the Kondo effect or inelastic electron tunneling (IET), where calculations suggested an IET origin due to vibrational modes.
   • Resonant peaks identified as shifting in accordance with Dirac point voltage, attributed to impurity density of states from the Co/graphene interaction.
   • Features that reverse in energy direction compared to the resonant peaks when gated, linked to ionization processes.
3. Defects in Graphene: Intrinsic defects exhibited behaviors similar to Co adatoms, displaying gate-induced ionization captured in ring-like structures in STS maps.

Implications and Future Prospects

The ability to ionize Co adatoms on graphene via controlled gating showcases the feasibility of dynamically tuning impurity states in graphene. This offers significant implications for the development of graphene-based chemical sensors and electronic devices where charge carrier density can be precisely modulated. Furthermore, the observation of large screening clouds suggests potential impacts on transport properties, identifying a key factor affecting mobility and conductivity in graphene systems.

Future research could explore the use of similar ionization techniques to modulate other types of adatoms or defects on graphene and related two-dimensional materials. Additionally, the incorporation of these findings into device architecture could lead to innovations in sensor technologies and the broader field of material science, where control at the atomic level is crucial for device optimization. The implications for electronic and spintronic applications, where magnetic transitions are involved, also remain open for exploration.

In conclusion, this work augments the understanding of impurity physics in graphene, merging theoretical insights with empirical measurements to advance both fundamental knowledge and practical application development in nanotechnology and material science.