[Paper Review] Charge transition levels of quantum emitters in hexagonal boron nitride

Zai‐Quan Xu, Noah Mendelson+4 more|arXiv (Cornell University)|Jun 30, 2019

Diamond and Carbon-based Materials Research4 citations

TL;DR

This study introduces a graphene-based charge transfer technique to experimentally determine charge transition levels (Eₜ) of quantum emitters in hexagonal boron nitride (hBN). By tuning the Fermi level of graphene via NMP functionalization, the authors identify that emitters with zero-phonon line (ZPL) wavelengths >600 nm have Eₜ levels above graphene’s Fermi level, enabling quenching and subsequent recovery upon doping control, thus quantifying Eₜ within ~0.3 eV precision.

ABSTRACT

Quantum emitters in layered materials are promising candidates for applications in nanophotonics. Here we present a technique based on charge transfer to graphene for measuring the charge transition levels ($ m E_t$) of fluorescent defects in a wide bandgap 2D material, and apply it to quantum emitters in hexagonal boron nitride (hBN). Our results will aid in identifying the atomic structures of quantum emitters in hBN, as well as practical applications since $ m E_t$ determines defect charge states and plays a key role in photodynamics.

Motivation & Objective

• To determine the charge transition levels (Eₜ) of fluorescent defects in hBN, which are critical for understanding defect charge states and photodynamic behavior.
• To develop a method for experimentally probing Eₜ using charge transfer to graphene in van der Waals heterostructures.
• To correlate Eₜ with emission wavelength and control quenching via tunable graphene Fermi level through NMP functionalization.
• To provide quantitative Eₜ values within hBN’s bandgap to guide atomic structure identification and improve device stability.
• To enable control over blinking and spectral diffusion in hBN-based single-photon emitters through electronic engineering of the host environment.

Proposed method

• Fabricated graphene-hBN heterostructures using PMMA-assisted wet transfer to enable controlled charge transfer to hBN defects.
• Used photoluminescence spectroscopy to measure zero-phonon lines (ZPLs) and phonon sidebands of quantum emitters in hBN.
• Employed environmental photoelectron yield spectroscopy (EPYS) to measure the Fermi level (E_F) of graphene relative to vacuum level (E_vac) in as-transferred and NMP-functionalized states.
• Functionalized graphene with N-methyl-2-pyrrolidone (NMP) to shift E_F upward by ~0.3 eV, confirming the shift via EPYS and Raman spectroscopy.
• Correlated quenching of ZPL emission (for λ > 600 nm) with Eₜ > E_F, and recovery after NMP functionalization, to infer Eₜ positions.
• Constructed an energy diagram showing Eₜ levels relative to E_F, E_vac, and hBN band edges, with Eₜ for 600 nm ZPL located ~3.6 eV above hBN valence band maximum.

Experimental results

Research questions

• RQ1 What is the charge transition level (Eₜ) of quantum emitters in hBN with ZPLs above 600 nm?
• RQ2 How does tuning the Fermi level of graphene via NMP functionalization affect the quenching of hBN quantum emitters?
• RQ3 Can charge transfer to graphene be used as a reliable method to probe Eₜ levels of defects in 2D materials?
• RQ4 What is the energy range of Eₜ levels that lie between the Fermi levels of as-transferred and NMP-functionalized graphene?
• RQ5 How do Eₜ levels correlate with photostability, blinking, and spectral diffusion in hBN emitters?

Key findings

• Emitters with ZPLs >600 nm are quenched by graphene due to Eₜ levels lying above the Fermi level (E_F) of as-transferred graphene.
• NMP functionalization of graphene shifts E_F upward by ~0.3 eV, restoring emission from emitters with ZPLs >600 nm, confirming Eₜ alignment with E_F.
• EPYS measurements show E_F at 4.7 eV for as-transferred graphene and 4.4 eV for NMP-functionalized graphene, confirming a ~0.3 eV upward shift.
• The charge transition level for a 600 nm ZPL emitter is located ~3.6 eV above the valence band maximum of hBN, consistent with Eₜ ≈ E_F1.
• The Eₜ levels of emitters with ZPLs >600 nm lie within a ~0.3 eV energy window between E_F of as-transferred and NMP-functionalized graphene.
• Raman spectroscopy independently confirms the p-type to slightly n-type transition in graphene after NMP functionalization, supporting the EPYS results.