Two-dimensional materials are interesting since they are atomic layer thin films that may be electrically pushed into vibration and function as nano-electromechanical resonators (NEMS) with characteristic resonance frequencies exceeding 100 MHz and typically ultrahigh quality characteristics at low temperatures.
When optical degrees of freedom are taken into account, NEMS can be developed into nano-opto-electro-mechanical systems (NOEMS), a family of hybrid solid devices designed to relate optical, electrical and mechanical degrees of freedom in nanoscales, particularly employing functional nano-materials.
Although vdW materials are renowned for developing phenomena like spin, valley and topological physics, research on NOEMS using van der Waals (vdW) heterostructures is scarce.
A universal approach has been discovered by researchers at Shanxi University and collaborators worldwide to simply and robustly build vdW heterostructures into an architecture that accommodates opto-electro-mechanical couplings in a single system.
Employing monolithically sculpted graphene/h-BN NOEMS as a platform, the researchers exhibited many functions, namely nano-mechanical resonators, vacuum channel diodes and ultra-fast thermo-radiators.
The optical readout of electric and magnetic field modulation of mechanical resonance in a CrOCl/graphene vdW NOEMS is also demonstrated, indicating that the addition of the vdW heterostructure to the NOEMS family has a lot of potential for developing new lab-on-a-chip devices.
To create arrays of suspended vdW heterostructures, holey h-BN was shaped using conventional lithography and plasma etching, followed by a dry transfer procedure employing pre-stacked few-layer vdW materials. At the end of the plasma etching process, groups of suspended vdW heterostructures were monolithically formed.
In this way, we can obtain a high yield of suspended vdW heterostructures, without the use of critical point drier, and this is crucial as we can now readily fabricate arrays of multi-terminal NOEMS, such as suspended Hall bars made of vdW multi-layers.
Dr. Xiaoxi Li, Lead Researcher, Shanxi University
Transverse voltage in a suspended vdW heterostructure is recognized to be critical to quantify during optical pumping/probing, such as valley Hall and quantum Hall phenomena (Figure 2), which had been a technological fault in suspended 2D materials. According to Dr. Li, at a suspension height of 300 nm and a suspended area of 5 μm2, the success rate exceeds 90%.
Thanks to the abundant library of 2D materials, we can therefore select and design all kinds of vdW heterostructures based NOEMS at our needs, for example, one can take advantage of the valley and spin degrees of freedom in transition metal dichalcogenides or in 2D magnetic materials to build devices with exotic opto-electro-mechanical couplings.
Dr. Tongyao Zhang, Study First Author, Shanxi University
“Furthermore, their functionalities can be selectively modulated by patterning the 2D stacks into photonic crystals, which opens up opportunities for novel NEOMS,” Dr. Zhang added.
Using the approach outlined above, multi-functionalities can be created in a single suspended vdW heterostructure, using an h-BN/graphene heterostructure as an example.
To begin with, the devices can be used as nanomechanical resonators, with resonance on the order of 0.1 GHz and a quality factor of 1000 at room temperature. Second, the entire device can function as a vertical vacuum channel diode when the lower surface of the suspended vdW heterostructure is set to graphene.
Thermionic emission of electrons from Joule heated graphene may be pushed out and collected by the bottom collector electrode, with on-off ratios reaching 105 and remaining unchanged after a total ionizing dosage of 3 Mrad (Si). Third, such a device can operate as thermo-radiators in the ultra-fast regime, with the wavelength of radiated light being controlled by the depth of the suspension cavity.
In a CrOCl/graphene vdW NOEMS, the scientists also showed an optical readout of electric and magnetic field tuning of mechanical resonance. At the magnetic phase transition in the system, a mechanical resonance frequency shift of roughly 0.8 MHz was discovered, indicating the possibility of opto-mechanical detection of complicated electromagnetic responses in vdW heterostructures.
Our approach is an exciting attempt of facile and robust fabrication protocol toward suspended vdW heterostructures, in principle, the proposed monolithically sculpted nano-opto-electromechanical vdW heterostructure system can be expanded to a wide variety of 2D materials, and can also be shaped into multi-terminal NOEMS.
Chengbing Qin, Professor and Lead Researcher, Shanxi University
The suspended vdW heterostructures thus aid in the exploration of a variety of intriguing features in both the classical and quantum regimes, and thus offer promise for future deployments of current nano-sensors.
Zhang, T., et al. (2022) A monolithically sculpted van der Waals nano-opto-electro-mechanical coupler. Light: Science & Applications. doi.org/10.1038/s41377-022-00734-7.