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Strain engineering of atomically-thin nanomembrane-based electromechanical devices

Final Report Summary - STRENGTHNANO (Strain engineering of atomically-thin nanomembrane-based electromechanical devices)

• a summary description of the project objectives
In STRENGTHNANO, strain engineering will be employed to modify the mechanical and electronic properties of graphene and other 2D materials. The main objective of the project can be divided in different tasks:
Fabrication of strain-tunable samples of graphene and other 2D materials.
Fabrication of freely suspended 2D materials.
Study of the strain dependence of the electrical and mechanical properties of 2D materials.
Electrical detection of the vibration of graphene-NEMS
Study of the strain dependence of the electrical properties of other 2D materials

• a description of the work performed since the beginning of the project,
A brief description of the work performed regarding each of the main objectives is presented in this section.

Fabrication of strain-tunable samples of graphene and other 2D materials

The project has advanced substantially towards the achievement of this goal. Several straining schemes have been studied and compared in order to find the most appropriate scheme for each experiment and a method to fabricate freely suspended 2D materials have been developed. The outcome of this part of the project is reflected by publications in peer-reviewed journals:
[1] van Veen et al. Graphene 2 (1) 13-17 (2013)
[2] Castellanos-Gomez et al. Nano Letters 13(11) 5361–5366 (2013)
[3] Plechinger et al. 2D Materials (2015, in press)
These different strategies are bending flexible substrates [1], wrinkling the lattice by buckling induced delamination [2] of exploiting the thermal expansion mismatch between the 2D material and the substrate [3].

Fabrication of freely suspended 2D materials

A new deterministic transfer process has been developed to fabricate freely suspended 2D materials [4]. We employed this newly developed technique to fabricate mechanical resonators with multilayer graphene [5] and with the semiconductor MoS2 [6]. This part of the project produced a high impact publications:
[4] Castellanos-Gomez et al. 2D Materials 1 011002 (2014)
[5] Singh et al. Nature Nanotechnology 9(10) 820–824 (2014)
[6] Castellanos-Gomez et al. Advanced Materials 25 (46) 6719–6723 (2013)

Electrical detection of the vibration of graphene-NEMS

The project also advanced towards the achievement of mechanical resonators with tunable electromechanical properties and an electrical read-out of their vibration [5]. Figure 1 shows an image of a few-layer graphene resonator capacitively coupled with a superconducting microwave resonator. The resulting system behave similarly to a conventional optomechanical system but the mechanical resonator has been substituted by a nanodrum which oscillates very close to its fundamental state (occupation state of few phonons at low temperature) and it is coupled to the radiation pressure from the microwave photons (instead of conventional cavities that employ visible light photons).
Apart from the electrical detection, we have implemented an optical detection read-out to detect the oscillation of mechanical resonators based on 2D materials. We have used this system to measure (for the first time) mechanical resonators based on single-layer MoS2.

Study of the strain dependence of the electrical and mechanical properties of 2D materials

This second part of the project also advanced significantly. Special focus has been paid to the strain tunability of the optoelectronic properties of atomically thin MoS2 (a 2D semiconductor) because of the recent interest in the scientific community (with many theoretical works recently published). Importantly, within STRENGTHNANO it has been experimentally demonstrated (by the first time) that non-uniform strain can be used to tune the optoelectronic properties and to control the exciton dynamics in 2D semiconductors [2].
For mechanical resonators, the electrostatic coupling with the gate electrode has been used to tune the mechanical resonance frequency (see Figure 1, right).

Study of the strain dependence of the electrical properties of other 2D materials

As pointed out in the Mid-term report, “…Due to the interest aroused in the scientific community about MoS2 and other 2D semiconductors, I have considered timely to focus more on these 2D semiconductor family instead of the Bismuth based chalcogenides…”
This change in focus has result in a high production of scientific results in the blooming field of 2D semiconductos (see next point). In particular (as pointed out in the previous point) STRENGTHNANO experimentally demonstrated that non-uniform strain can be used to tune the optoelectronic properties and to control the exciton dynamics in 2D semiconductors [2].

• a description of the main results achieved so far,
Development of a new transfer technique
Demonstration of local strain engineering in 2D semiconductors
Coupling of graphene-based resonator to a superconducting microwave cavity
Demonstration of single-layer MoS2 mechanical resonators
Exploration of novel 2D semiconductors like black-phosphorus
Development of new straining methods for strain engineering in 2D materials

Publication achievements:
22 peer-reviewed articles + 1 invited book chapter (and another one in press)

• the expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far).
STRENGTHNANO has addresses key fundamental questions, important for both the advance of the research on nanomaterials and for future applications of two-dimensional semiconductors. This research opens the door to design straintronic devices whose functionality can be engineer by exploiting the outstanding strain tunability of the optoelectronic properties of 2D semiconductors. The field of straintronics is a blooming and promising new avenue to achieve devices with extra functionalities not possible with conventional semiconductors.