Very Long Term Stabilization of a 2D Magnet down to the Monolayer for Device Integration

Abstract

2D materials have recently demonstrated a strong potential for spintronic applications. This has been further reinforced by the discovery of ferromagnetic 2D layers. Nevertheless, the fragility of many 2D magnetic materials to ambient conditions has so far hindered their faster characterization and integration into devices. We report here on a simple large-scale method that allows to stabilize strongly air sensitive materials, such as CrBr3, down to the monolayer limit with ultrathin barriers grown by atomic layer deposition (ALD). We focus on MgO as a passivation layer to additionally serve as tunnel spin injection barrier for spintronic applications. We develop a special removable combined protection–encapsulation stack to better preserve 2D material and MgO barrier qualities during device fabrication. This scheme allows to observe 2D ferromagnet stability over one year of air exposure and to demonstrate CrBr3 successful integration into vertical devices. Overall, these results highlight an efficient way to handle these materials in ambient conditions, unlocking possibilities to fasten their advanced characterization and ease their integration into devices.

Optical Harmonic Generation in 2D Materials

“…However, the fragility of many 2D materials to ambient conditions has so far hindered their integration into practical devices. [ 139 ] Researchers have developed various ways to stabilize air sensitive 2D materials. [ 140,141 ] For instance, black phosphorus degrades with the environment [ 142 ] and has been preserved through encapsulation with Al 2 O 3 layer deposition, [ 143 ] capping of hBN layers, [ 144 ] and encapsulation with PMMA.…”

“…[ 142 ] Similarly, CrBr 3 , an air sensitive 2D magnetic material is protected with ultrathin barriers of MgO grown by atomic layer deposition (ALD). [ 139 ] Apart from environmental conditions, some 2D materials have shown degradation behavior toward strong electromagnetic fields such as high‐powered lasers. [ 140 ] For instance, high powered lasers directly ablate PdSe 2 thin films, whereas, lower laser power locally transform PdSe 2 into a metallic phase.…”

Ambient-Stable Two-Dimensional CrI3 via Organic-Inorganic Encapsulation

“…Despite the growing interest in 2D magnets, one of the principal factors limiting their study and applications is their high chemical reactivity. − For example, the prototypical 2D magnetic material CrI 3 photocatalytically decomposes within seconds upon ambient exposure . The ambient reactivity of transition metal halides, such as CrI 3 , CrBr 3 , VI 3 , and related magnetically ordered 2D materials, has limited fundamental studies to highly idealized conditions or sample handling geometries that are incompatible with scalable manufacturing. Early attempts to mitigate the ambient reactivity of CrI 3 have included poly­(methyl methacrylate) (PMMA) coatings, electron-beam evaporation of alumina, and sandwiching between micromechanically exfoliated hBN crystals .…”

“…Consequently, direct ALD of oxides on vdW materials often yields porous films with large roughness, resulting in poor encapsulation . To circumvent this issue, noncovalent organic seeding layers, such as N -methyl pyrrolidone (NMP), perylenetetracarboxylic dianhydride (PTCDA), and related perylene derivatives, have been employed to initiate ALD growth, resulting in pinhole-free metal oxide films that have facilitated ambient processing and device fabrication for graphene, InSe, and TMDCs. , On the other hand, initial ALD attempts on the transitional metal halide CrBr 3 have utilized ozone exposure to activate the surface prior to growth. While this surface activation approach does provide reactive sites for conformal ALD, it also leads to chemical changes to the top surface that likely compromise physical properties, particularly in the monolayer limit.…”

Degradation Chemistry and Kinetic Stabilization of Magnetic CrI3

“…13,35−37 The current CrI 3 stabilization strategies mainly rely on the implementation of a multistep physical encapsulation method including protection via a polymer, a hexagonal boron nitride (hBN) layer, and atomic layer deposition of metallic oxide (e.g., Al 2 O 3 ). 13,[17][18][19][20][21][22]27,37 Physical encapsulation may offer the advantage of direct device integration but lacks high scalability, solution processability, and the possibility of additional chemical functionalization, which can be achieved by using a chemical stabilization method. The latter strategy has already been exploited to stabilize air-sensitive 2D materials.…”