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Unlocking new physics in controllably strained two-dimensional materials

Periodic Reporting for period 3 - Strained2DMaterials (Unlocking new physics in controllably strained two-dimensional materials)

Reporting period: 2018-11-01 to 2020-04-30

The overarching goal of Strained2DMaterials is to use strain engineering as an enabling tool to study previously inaccessible or hard-to-study phenomena in two-dimensional atomic crystals (e.g.: graphene, bilayer graphene, and monolayer transition metal dichalcogenides). We intend to: (i) Demonstrate the dominant effect of commonly neglected flexural phonons on electrical transport, thermal conductivity, and mechanical properties of suspended graphene. (ii) Observe the effects of “pseudomagnetic fields” in controllably strained graphene and TMDCs. These observations may lead to realization of devices employing Quantum Hall effect but operating at zero magnetic field. (iii) Develop the techniques to control and tune excitons in TMDC material using uniform and non-uniform strain fields. These techniques may be used to create efficient photoconverting devices and/or building blocks for excitonic electronics.
We designed and implemented two new approaches to induce highly non-uniform strain in 2D materials. In one approach, a suspended 2D material is placed inside an optical cryostat and is controllably indented from the bottom by a tip of a homebuilt atomic force microscope. In this approach, highly non-uniform, tunable, and large strain field is induced in a 2D material in vacuum and at low temperatures. In the second approach, a compressed gas is used to pressurize a 2D material suspended over a hole of controllably shape. A tunable non-uniform strain (that is smaller than that in the first approach) is induced. Using these schemes, we investigate the impact of non-uniform strain on excitons in TMDCs. We showed that exciton funnelling, the effect which was thought to dominate their behaviour, is very small. In contrast, we demonstrated that funnelling of charge carriers towards the point of the highest strain followed by their binding to neutral excitons forming the so-called “trions” dominates the photophysics of non-uniformly strained devices. We also developed a theoretical approach to analyse non-uniformly strained devices and suggested a scheme to use such devices for photoconversion or for observation of correlated physics of excitons.

We developed an approach to chemically modify 2D materials and to investigate strain and doping that results from such modification. We showed that defects in 2D materials are chemically active. We demonstrated approaches to study the properties of such defects and to modify them in situ.

Together with the group of Saikat Ghosh (IIT Kanpur), we investigated 2D material membranes towards applications in NEMS (nanoelectromechanical systems). Specifically, we studied graphene resonators deposited on a much larger and heavier SiNx membrane. We demonstrated widely tunable, broad bandwidth, and high gain all-mechanical motion amplifiers based on graphene/silicon nitride (SiNx) hybrids. In these devices, a tiny motion of a large-area SiNx membrane is transduced to a much larger motion in a graphene drum resonator coupled to SiNx. We obtain a displacement power gain of 38 dB and demonstrate 4.7 dB of squeezing, resulting in a detection sensitivity of 3.8 fm/Hz^0.5 close to the thermal noise limit of SiNx. Furthermore, we discovered that strong coupling to mechanically non-linear graphene induces large non-linearity in normally mechanically linear SiNx. The induced non-linearity in SiNx allows us to observe a range of behavior previously unseen in SiN resonators including frequency comb generation and Arnold tongues.
"The main achievements of the project so far are as follows:

1) Discovery of a non-linear Hooke's law in crumpled graphene
2) Experimental discovery of exciton/trion funneling in non-uniformly strained TMDCs and single quantum emitters in non-uniformly strained hBN
3) Development of NEMS devices based on coupled SiNx/graphene hybrids: ultrasensitive mechanical amplifiers and devices with tunable nonlinearities
4) Development of approaches to non-uniform strain engineering
5) Development of approaches to chemically engineer 2D materials

Until the end of the project, we plan to achieve the following main goals:

1) Demonstration of strain-controlled 2D ""phononic crystals""
2) Demonstration of tunable pseudomagnetic fields in TMDC and graphene. Exploration of the emergent physical phenomena related to these fields.
3) Development of techniques to probe surface contamination of 2D materials, towards the creation of devices with well-characterized mechanical properties."
Strain Engineering of 2D materials