High Temperature Quantum Emission from Covalently Functionalized van der Waals Heterostructures

TL;DR

This work targets the limited operating temperature of defect-emission single-photon sources in 2D TMDs by combining WSe$_2$/graphite heterostructures with covalent 4-NBD functionalization. The covalent modification opens a graphene bandgap within the WSe$_2$ gap and couples to midgap defect states, enabling bright, high-purity SPE up to $T=90\ \mathrm{K}$ and preserving single-photon integrity up to $T=115\ \mathrm{K}$ with emission near $800\ \mathrm{nm}$; lifetimes shorten and spectral isolation improves without cavities. Raman and DFT analyses support a top-surface covalent attachment that localizes carriers and introduces resonant pathways. The results demonstrate a scalable, molecule- and interface-engineered route to high-purity, elevated-temperature quantum emission in 2D heterostructures, with potential for device integration and tunable emitter properties. ${}$

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are attractive nanomaterials for quantum information applications due to single photon emission (SPE) from atomic defects, primarily tungsten diselenide (WSe2) monolayers. Defect and strain engineering techniques have been developed to yield high purity, deterministically positioned SPE in WSe2. However, a major challenge in application of these techniques is the low temperature required to observe defect-bound TMD exciton emission, typically limiting SPE to T<30 K. SPE at higher temperatures either loses purity or requires integration into complex devices such as optical cavities. Here, 2D heterostructure engineering and molecular functionalization are combined to achieve high purity (>90%) SPE in strained WSe2 persisting to over T=90 K. Covalent diazonium functionalization of graphite in a layered WSe2/graphite heterostructure maintains high purity up to T=90 K and single-photon source integrity up to T=115 K. This method preserves the best qualities of SPE from WSe2 while increasing working temperature to more than three times the typical range. This work demonstrates the versatility of surface functionalization and heterostructure design to synergistically improve the properties of quantum emission and offers new insights into the phenomenon of SPE from 2D materials.

Figures (6)

• Figure 1: (a) Optical microscope image and side-view schematic of the functionalized heterostructure. A layer of polymethyl methacrylate (PMMA) is spin-coated onto a dielectric substrate of silicon/silicon dioxide (Si/$\text{SiO}_{\text{2}}$). Monolayer (1L) $\text{WSe}_{\text{2}}$ (red) is then transferred onto PMMA and indented with atomic force microscopy (AFM). Graphite (black) is then transferred over part of this monolayer. The white dashed box outlines the area between graphite and $\text{WSe}_{\text{2}}$ that has been nano-squeegeed together. This entire heterostructure is functionalized with 4-NBD, which leaves nitrophenyl groups either covalently or non-covalently adsorbed to the surface. (b) Spectra of the same squeegeed graphite-covered indent location before (green) and after (purple) functionalization, showing an increase in emitter intensity of the narrow emission feature at 800 nm for the same amount of laser excitation power. The laser excitation wavelength used here and throughout the work is 532 nm. (c) $g^{(2)}(\tau)$ measured for the emitter highlighted in (b) prior to functionalization. The emitter lifetime is $\tau = 6.48 \pm 0.01$ ns. (d) $g^{(2)}(\tau)$ measured for the same emitter after functionalization, where the lifetime is now $\tau=3.95\pm 0.08$ ns, a 40% decrease in emitter lifetime.
• Figure 2: Results of a $\text{WSe}_{\text{2}}$/graphite heterostructure functionalized with 4-NBD; here the graphite thickness is 1.6 nm. (a) Emission intensity map of the heterostructure at $T=1.6$ K. The red dashed line outlines the $\text{WSe}_{\text{2}}$ area. The black dashed box within this area outlines the portion of graphite and $\text{WSe}_{\text{2}}$ that are squeegeed together. The relatively bright emission surrounding the squeegeed area is due to the flourescence of residue such as transfer polymer that was pushed out from between the layers rosenberger2018nano. (b) Spectrum of two indent locations, one where just $\text{WSe}_{\text{2}}$ is functionalized with 4-NBD (purple) and one where the graphite heterostructure is functionalized (yellow). The yellow spectrum has a strong emission line around 800 nm, which is shown to be SPE at low temperature in (c). Figures (d)-(f) show the same data at $T=90$ K. In (e), the purple spectrum of the same indent as (b) shows that localized emission on the functionalized $\text{WSe}_{\text{2}}$ is gone, whereas strong emitters remain on the graphite heterostructure side (yellow). This emitter is still high purity SPE even at $T=90$ K.
• Figure 3: (a) Raman spectra of graphite after 4-NBD functionalization, one with 1.6-nm thickness (yellow) and the other with 8 nm thickness (purple). Both spectra are normalized to the graphite G peak. The D peak is detectable on the thin graphite with a resulting D/G peak ratio of about 20%, indicating disruptions on the graphite surface due to covalent bonds. (b,c) Images of possible nitrophenyl (NPh) position in layered heterostructure configurations. In (b), NPh is covalently attached to graphene but located between the graphene and $\text{WSe}_{\text{2}}$. In (c), NPh is covalently bonded to the top of the graphene. This configuration is more stable and consistent with experimental findings from (a).
• Figure 4: Results of a heterostructure with graphite transferred directly on top of functionalized monolayer $\text{WSe}_{\text{2}}$. (a) OM of a functionalized monolayer. (b) OM of this same monolayer with graphite (outlined in yellow) transferred on top. (c) Intensity map showing maximum peak intensity of the functionalized monolayer without graphite. (d) Intensity map of the functionalized monolayer with graphite (outlined in yellow) transferred on top. Here, the graphite suppresses nearly all emission from both flat and strained areas. Localized emission from indents is very weak and disappears by $\sim10K$.
• Figure 5: The electronic band structure of monolayer $\text{WSe}_{\text{2}}$ (blue) with a single selenium vacancy (green), pristine graphene (black), and covalently functionalized graphene (black) with an additional energy level introduced by nitrophenyl (red). The valence band maximum is shifted to zero and is shown by the black dashed line.