Method for preparing clean insulating single or few sheets of topological insulators using an ionic liquid

Abstract

A method to produce high quality single or a few atomic layers thick samples of a topological insulating layered dichalcogenide. The overall process involves grinding layered dichalcogenides, adding them to an ionic liquid, and then using a mechanical method to cause intercalation of the ionic liquid into the van der Waals (VDW) gap between the layers of the metal chalcogenide.

Claims

1. A method for preparing clean, insulating sheets of a topological insulator, comprising: adding a layered dichalcogenide to an ionic liquid, wherein the dichalcogenide comprises bismuth, and wherein the ionic liquid comprises 1,2-dimethyl-3-octylimidazolium paired with bis(trifluoromethanesulfonyl)imide; using a mechanical method to cause intercalation of the ionic liquid into a van der Waals gap between the layers of the dichalcogenide; and continuing the mechanical method to cause an individual sheet of the layered dichalcogenide to break apart or to cause a few sheets of the layered dichalcogenide to break apart with no bismuth remaining between the layers.

2. The method of claim 1, wherein the layered dichalcogenide is Bi.sub.2X.sub.3, where X is Se or Te.

3. The method of claim 1, wherein the mechanical method comprises a vibrational interaction.

4. The method of claim 1, wherein the mechanical method comprises micro stirring and sonication of less than 20 joules of energy.

5. The method of claim 1, wherein the mechanical method comprising stirring and applying heat.

6. Insulating sheets of a topological insulator made by the method, comprising: adding a layered dichalcogenide to an ionic liquid, wherein the dichalcogenide comprises bismuth, and wherein the ionic liquid comprises 1,2-dimethyl-3-octylimidazolium paired with bis(trifluoromethanesulfonyl)imide; using a mechanical method to cause intercalation of the ionic liquid into a van der Waals gap between the layers of the dichalcogenide; and continuing the mechanical method to cause an individual sheet of the layered dichalcogenide to break apart or to cause a few sheets of the layered dichalcogenide to break apart with no bismuth remaining between the layers.

7. The insulating sheets of a topological insulator of claim 6, wherein the layered dichalcogenide is Bi.sub.2X.sub.3, where X is Se or Te.

8. The insulating sheets of a topological insulator of claim 6, wherein the mechanical method comprises a vibrational interaction.

9. The insulating sheets of a topological insulator of claim 6, wherein the mechanical method comprises micro stirring and sonication of less than 20 joules of energy.

10. The insulating sheets of a topological insulator of claim 6, wherein the mechanical method comprising stirring and applying heat.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the structure of Bi.sub.2X.sub.3, where X is Se or Te. The dark atoms are Bi, and the light atoms are Se or Te. VDW gaps indicate where the shearing of the layered material will occur.

(2) FIG. 2 is a schematic representation of the exfoliation reaction.

(3) FIGS. 3A-3D show a photographic presentation of reaction and purification steps. FIG. 3A shows pure ionic liquid. FIG. 3B shows ionic liquid after micro-stirring with Bi.sub.2X.sub.3. FIG. 3C shows after allowing to settle, the volume of the Bi.sub.2X.sub.3 is significantly greater (it was just a thin layer of metal dust initially). FIG. 3D shows the remaining material after centrifugation but prior to resuspension in an organic solvent.

(4) FIG. 4A shows the chemical structure for a 1,2-dimethyl-3-octylimidazolium cation. FIG. 4B shows the chemical structure for a bis(trifluoromethanesulfonyl)imide anion. This cation and anion can be paired to use as an ionic liquid.

(5) FIG. 5 shows the general structure of some possible cations to be used in the ionic liquid.

(6) FIG. 6 shows the general structure of some possible anions to be used in the ionic liquid.

(7) FIGS. 7A and 7B show TEM results of the topological insulator prepared comprising Bi.sub.2Se.sub.3. FIG. 7A shows a lower magnification view, and FIG. 7B shows a higher magnification complement. FIG. 7C shows an energy dispersive spectrum of the topological insulator prepared comprising Bi.sub.2Se.sub.3.

(8) FIGS. 8A and 8B show TEM results of the topological insulator prepared comprising Bi.sub.2Te.sub.3. FIG. 8A shows a lower magnification view, and FIG. 8B shows a higher magnification complement. FIG. 8C shows an energy dispersive spectrum of the topological insulator prepared comprising Bi.sub.2Te.sub.3.

(9) FIG. 9 shows SEM size analysis of sheets prepared using Bi.sub.2Se.sub.3.

(10) FIG. 10 shows SEM size analysis of sheets prepared using Bi.sub.2Te.sub.3.

DETAILED DESCRIPTION OF THE INVENTION

(11) The overall goal of the present invention is to produce high quality single or a few atomic layers thick samples of the topological insulating layered dichalcogenide Bi.sub.2X.sub.3, where X is Se or Te. The overall process involves first grinding the layered dichalcogenides, adding them to an ionic liquid, and then using a mechanical method to cause intercalation of the ionic liquid into a van der Waals (VDW) gap between the layers of the metal chalcogenide. FIG. 1 shows the structure of the Bi.sub.2X.sub.3 compounds, with arrows indicating the VDW gap.

(12) According to the present invention, an ionic liquid is combined with a vibrational interaction such as micro stirring and weak sonication (less than 20 joules of energy as opposed to typical mega-joules or energy), or simple heat and stirring to initiate intercalation of the ionic liquid into layered topological dichalcogenide insulators. Although this work focuses on the Bi.sub.2X.sub.3 topological insulators, it is likely applicable to other types of layered topological insulators, such as Bi.sub.2Sb.sub.2, or SmB.sub.6. The chemical pathway leading to exfoliation is shown graphically in FIG. 2. In the structural schematic of Bi.sub.2X.sub.3, the gray lines indicate sheets of X, while the black lines indicate sheets of Bi. Each unit cell of Bi.sub.2X.sub.3 is composed of two sheets of Bi sandwiched between 3 sheets of X. The van der Waals gap where intercalation occurs is where two x layers meet. The dark circles represent the defect Bi atoms between sheets shoring out the insulating nature of the material. In the reaction between the Bi.sub.2X.sub.3 and ionic liquid, the ionic liquid is added, and then the solution is sonicated/blended. Initially, the ionic liquid inserts itself between the layers into the van der Waal gaps. At completion, the individual sheets of the layered material come apart completely in the solution. These single sheets are then defect free and no longer have Bi between layers shorting out the insulating nature of the material.

(13) These solutions of layered material and the ionic liquid are allowed to react for 24 hours to several days, and then centrifuged and washed with acetonitrile or other solvent in which the ionic liquid is soluble but the layered material is not. After centrifugation, the remaining material is then re-suspended in an organic solvent. A small aliquot of this is then subjected to additional cleaning in an organic solvent prior to characterization by TEM to confirm the formation of clean sheets of the material. FIGS. 3A-3D show a photographic presentation of the different reaction and purification steps. The ionic liquid is clear color less fluid (FIG. 3A), then the dichalcogenide is added and sonicated (FIG. 3B). After this, the solution appears black with the suspended layered material (FIG. 3C). Methanol (MeOH) is then added and the solution is centrifuged (FIG. 3D) and the solid material washed several times to remove the ionic liquid.

(14) For the ionic liquid, a tri-substituted imidazolium cation (1,2-dimethyl-3-octylimidazolium (FIG. 4A)) paired with a hydrophobic anion, bis(trifluoromethanesulfonyl)imide (FIG. 4B), can be used. Using a hydrophobic ionic liquid, especially one with a flat aromatic positively charged imidazolium ring allows for a non-aqueous method for performing the initial intercalation that results in ultimately, exfoliation. Many other ionic liquids based on general cations (FIG. 5) and anions (FIG. 6) will also work.

(15) FIGS. 7A and 7B show the TEM results of the topological insulator prepared using Bi.sub.2Se.sub.3. FIG. 7A shows a lower magnification view, while FIG. 7B shows a higher magnification complement. Likewise FIGS. 8A and 8B show the TEM results of the topological insulator prepared using Bi.sub.2Te.sub.3. FIG. 8A shows a lower magnification view, while FIG. 8B shows a higher magnification complement. Higher resolution scans show clean sheets of the materials. The TEM images were obtained using a Nion UltraSTEM 200, a microscope that has the ability to resolve 80 pm details, operating at 60 keV accelerating voltage. FIGS. 7C and 8C show an energy dispersive spectrum showing the elemental composition of the material (using Bi.sub.2Se.sub.3 for FIG. 7C and using Bi.sub.2Te.sub.3 for FIG. 8C).

(16) FIG. 9 shows the SEM size analysis of sheets prepared using Bi.sub.2Se.sub.3. As shown in FIG. 9, the size of the sheets is about 38 sq. microns. FIG. 10 shows the SEM size analysis of sheets prepared using Bi.sub.2Te.sub.3. As shown in FIG. 10, the size of the sheets is about 36 sq. microns.

(17) The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles a, an, the, or said, is not to be construed as limiting the element to the singular.