An official website of the United States government
Uniaxial strain has been widely used as a powerful tool for investigating and controlling the properties of quantum materials. However, existing strain techniques have so far mostly been limited to use with bulk crystals. Although recent progress has been made in extending the application of strain to two-dimensional van der Waals (vdW) heterostructures, these techniques have been limited to optical characterization and extremely simple electrical device geometries. Here, we report a piezoelectric-based in situ uniaxial strain technique enabling simultaneous electrical transport and optical spectroscopy characterization of dual-gated vdW heterostructure devices. Critically, our technique remains compatible with vdW heterostructure devices of arbitrary complexity fabricated on conventional silicon/silicon dioxide wafer substrates. We demonstrate a large and continuously tunable strain of up to −0.15% at millikelvin temperatures, with larger strain values also likely achievable. We quantify the strain transmission from the silicon wafer to the vdW heterostructure, and further demonstrate the ability of strain to modify the electronic properties of twisted bilayer graphene. Our technique provides a highly versatile new method for exploring the effect of uniaxial strain on both the electrical and optical properties of vdW heterostructures and can be easily extended to include additional characterization techniques.
more »
« less
Interlayer registry effects on the electronic and piezoelectric properties of transition metal dichalcogenide bilayers
Transition metal dichalcogenides (TMDC) are currently drawing significant interest from the scientific community as 2D materials that have intrinsically semiconducting bandgaps. One additional advantage of TMDCs for discovering and developing materials with novel electronic, electromechanical, or optoelectronic properties is that both layer composition and registry can be readily tailored. To understand how such tailoring can expand the range of properties, here we used density functional theory calculations to determine the electronic structure and piezoelectric properties of bilayer TMDC heterostructures based on MoX2 and WX2, where X can be S, Se, or Te. For identical layers with no misorientation with respect to one another, we find that the registry of the two layers can change the bandgap type (direct vs indirect), as well as its value (by ≈0.25 eV). We report similar conclusions for bilayer heterostructures in which the composition of the two layers is different. Interlayer registry also has a pronounced effect on piezoelectric properties as the piezoelectric coefficients of the two layers either nearly cancel each other or add up to yield enhanced values for the associated TMDC bilayer heterostructures. These results may serve as a guide for enhancing electronic and piezoelectric properties by stacking TMDC layers.
more »
« less
- Award ID(s):
- 2119281
- PAR ID:
- 10502737
- Publisher / Repository:
- American Vacuum Society
- Date Published:
- Journal Name:
- Journal of Vacuum Science & Technology A
- Volume:
- 42
- Issue:
- 3
- ISSN:
- 0734-2101
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Twisting or sliding two-dimensional crystals with respect to each other gives rise to moiré patterns determined by the difference in their periodicities. Such lattice mismatches can exist for several reasons: differences between the intrinsic lattice constants of the two layers, as is the case for graphene on BN; rotations between the two lattices, as is the case for twisted bilayer graphene; and strains between two identical layers in a bilayer. Moiré patterns are responsible for a number of new electronic phenomena observed in recent years in van der Waals heterostructures, including the observation of superlattice Dirac points for graphene on BN, collective electronic phases in twisted bilayers and twisted double bilayers, and trapping of excitons in the moiré potential. An open question is whether we can use moiré potentials to achieve strong trapping potentials for electrons. Here, we report a technique to achieve deep, deterministic trapping potentials via strain-based moiré engineering in van der Waals materials. We use strain engineering to create on-demand soliton networks in transition metal dichalcogenides. Intersecting solitons form a honeycomb-like network resulting from the three-fold symmetry of the adhesion potential between layers. The vertices of this network occur in bound pairs with different interlayer stacking arrangements. One vertex of the pair is found to be an efficient trap for electrons, displaying two states that are deeply confined within the semiconductor gap and have a spatial extent of 2 nm. Soliton networks thus provide a path to engineer deeply confined states with a strain-dependent tunable spatial separation, without the necessity of introducing chemical defects into the host materials.more » « less
-
Building on discoveries in graphene and two-dimensional (2D) transition metal dichalcogenides, van der Waals (VdW) layered heterostructures—stacks of such 2D materials—are being extensively explored with resulting new discoveries of novel electronic and magnetic properties in the ultrathin limit. Here, we review a class of naturally occurring heterostructures—the so-called misfits—that combine disparate VdW layers with complex stacking. Exhibiting remarkable structural complexity and diversity of phenomena, misfits provide a platform on which to systematically explore the energetics and local bonding constraints of heterostructures and how they can be used to engineer novel quantum fabrics, electronic responsiveness, and magnetic phenomena. Like traditional classes of layered materials, they are often exfoliatable and thus also incorporatable as units in manually or robotically stacked heterostructures. Here, we review the known classes of misfit structures, the tools for their single crystal and thin film synthesis, the physical properties they exhibit, and the computational and characterization tools available to unravel their complexity. Directions for future research are also discussed.more » « less
-
Abstract The transition metal selenides M1+ySe2(M = V, Ti) have intriguing quantum properties, which make them target materials for controlling properties by thinning them to the ultrathin limit. An appropriate approach for the synthesis of such ultrathin films is by molecular beam epitaxy. Here, it is shown that such synthesized V‐ and Ti‐Se2films can undergo a compositional change by vacuum annealing. Combined scanning tunneling and photoemission spectroscopy is used to determine compositional and structural changes of ultrathin films as a function of annealing temperature. Loss of selenium from the film is accompanied by a morphology change of monolayer height islands to predominantly bilayer height. In addition, crystal periodicity and atomic structure changes are observed. These changes are consistent with a transition from a layered transition metal dichalcogenide (TMDC) to ordered intercalation compounds with V or Ti intercalated in between two layers of their respective TMDCs. These observations may clear up misconception of the nature of previously reported high‐temperature grown transition metal selenides. More significantly, the demonstrated control of the formation of intercalation compounds is a key step toward modifying properties in van der Waals systems and toward expanding material systems for van der Waals heterostructures.more » « less
-
Complex oxides and semiconductors exhibit distinct yet complementary properties owing to their respective ionic and covalent natures. By electrically coupling complex oxides to traditional semiconductors within epitaxial heterostructures, enhanced or novel functionalities beyond those of the constituent materials can potentially be realized. Essential to electrically coupling complex oxides to semiconductors is control of the physical structure of the epitaxially grown oxide, as well as the electronic structure of the interface. Here we discuss how composition of the perovskite A- and B- site cations can be manipulated to control the physical and electronic structure of semiconductor – complex oxide heterostructures. Two prototypical heterostructures, Ba1-xSrxTiO3/Ge and SrZrxTi1-xO3/Ge, will be discussed. In the case of Ba1-xSrxTiO3/Ge, we discuss how strain can be engineered through A-site composition to enable the re-orientable ferroelectric polarization of the former to be coupled to carriers in the semiconductor. In the case of SrZrxTi1-xO3/Ge we discuss how B-site composition can be exploited to control the band offset at the interface. Analogous to heterojunctions between compound semiconducting materials, control of band offsets, i.e. band-gap engineering, provide a pathway to electrically couple complex oxides to semiconductors to realize a host of functionalities.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1116/6.0003264
