Spin-quenching in molecule-transition-metal-dichalcogenide heterostructure through inverse proximity effect

Abstract

A functional heterostructure is central to integrated circuitry in quantum photonics, optoelectronics, neuromorphic computing, spintronics, and straintronics. Recently, heterostructures combining 2D magnets and nonmagnetic transition metal dichalcogenides (TMDs) have been explored. In these, electron and hole wavefunctions are localized in 2D magnets but delocalized in TMDs. When combined, a proximity induced magnetic inter layer exciton can emerge, with energy differing by 20 to 30 meV from intra layer excitons and being two orders of magnitude darker, making it hard to detect and functionalize. Using a high fidelity ab initio many body diagrammatic approach, we show that functionality can be significantly enhanced in a transition metal molecule TMD interface. The molecular exciton exhibits charge transfer character and is extended, unlike the localized Frenkel excitation in 2D magnets. Moreover, the degree of localization and magnetic moment can be tuned by varying the molecular orientation relative to the TMD. This changes the proximity to the magnetic ion, altering screening and enabling a pathway to quench the ion's spin moment. This inverse proximity effect tunes the energies, spin states, and brightness of molecular and inter layer magnetic excitons, a mechanism absent in 2D magnet TMD systems. We also identify conditions under which the interlayer exciton becomes well separated and brighter than intra layer excitons, making it promising for protocols that probe and manipulate magnetic excitonic states.

Figures (4)

• Figure 1: The simulated crystal structures of the different VO(dmit)$_{2}$-TMD heterostructures are shown. The molecule (shown in the middle separately) is rotated at different angles with respect to the TMD layer. Two cases are shown with the angle between the layers respectively at 0° and 120°. RGB is used to represent the standard $abc$ crystallographic directions.
• Figure 2: Partial density of states (DOS) of the different heterostructures are shown. The DOS are shown from two levels of the theory: LDA and $\mathrm{QS}G\hat{W}$. Partial DOS for four different heterostructure configurations are projected on the molecular layer containing Carbon, Oxygen, Sulfur, Vanadium/Molybdenum (transition metal) and the WSe$_{2}$ layer and the $\uparrow$ and $\downarrow$ spins of the transition metal element. The change in bandgap, and especially $\mathrm{QS}G\hat{W}$'s reordering/realignment of the frontier orbitals has a pronounced effect on the $dd$ splittings and the excitons.
• Figure 3: Contrast between a Frenkel exciton (CrI$_3$, left) and a charge-transfer exciton (VO(dmit)$_{2}$, right). The bottom parts depict the spatial (real-space) extent of the excitons. For CrI$_3$ most of the exciton resides on a single Cr site (electron and hole on the same site). This is shown as blue in the donut plot in the top left panel. This magnetic exciton is highly localized reflecting its Frenkel character, extending only about $\sim$0.5 nm in real space. The charge-transfer exciton on the right extends over the entire molecule $\sim$2.5 nm and has a vanishingly small onsite character. This exciton has two roughly equal components: V hole coupled to ligand electrons (purple) and ligand hole coupled to ligand electrons (orange).
• Figure 4: The macroscopic dielectric response $\epsilon_2$ are plotted for different structural configurations of V (a,b) and Mo (c,d). 100 and 010 refer to the x and y components of the longitudinal elements of $\epsilon_2$. Series of intra-layer and inter-layer excitonic peaks are observed. Molecular intra-layer, WSe$_{2}$ intra-layer and inter-layer excitons are noted respectively as M, W and I. A uniform optical broadening of only 1 meV is used at all energies and optical spectra are plotted in log scale to reveal all the weaker excitonic structures. In the horizontal bar plots the decomposition of the excitonic wavefunctions into different intra- and inter-layer components are shown. In the legend for the bar plots, 'onsite' resolves the component of the exciton wavefunction that originates from purely atom-local onsite (Frenkel) transitions. It only makes an extremely weak appearance in the molecular 1.33 eV exciton. The smallness of this component is strikingly different from the 2D magnet/WSe$_2$ systems, and indicates these excitons have essentially no Frenkel character. WSe$_{2}$ primarily hosts intersite transitions and can not have much of onsite transitions and this implies the green color is nearly absent from all bar plots.The 'intersite' component links electron-hole pairs located at different sites. This includes optical transitions connecting the magnetic element and its ligands within the molecule (orange); or transitions between W and Se at different sites (red). VO(dmit)$_{2}$$^-$WSe$^+$ and VO(dmit)$_{2}$$^+$WSe$^-$ refer to excitons where the hole resides on the molecule and the electron in WSe$_2$, or vice versa. The bar plots show that to a good approximations excitons can be classed as purely WSe$_2$ (W), purely molecular (M), or purely interlayer (I).