image: a, The process of holes (1 μm ×3 μm in size, for example) patterning in the spacer h-BN (200-400 nm in thickness) by reactive ion etching (RIE). b, Transferring the holed h-BN onto the surface of Au (or graphite) modulating electrode by PPC (Propylene-Carbonate). c, Completely removing the residual PPC by vacuum annealing at 350℃ for 60 min. d-f, Transferring the h-BN/2D functional materials (Graphene, MoS2, WSe2, and etc.) stacks onto the surface of the holed h-BN/Au stack using a dry transfer method, and annealing at 350℃ for 60 min. g, Cr/Au (5 nm/200 nm) electrodes are deposited using thermal evaporation in the etched trenches followed by standard electron beam lithography (EBL). h, Another RIE process to pattern the emitter. Electron beam resists are removed by solvent. i, Enlarge view of the NOEMS coupler in h. This process gives over 90% sample yield, which is very reliable and without the requirement of the critical point drying procedure. view more
Credit: by Tongyao Zhang, Hanwen Wang, Xiuxin Xia, Ning Yan, Xuanzhe Sha, Jinqiang Huang, Kenji Watanabe, Takashi Taniguchi, Mengjian Zhu, Lei Wang, Jiantou Gao, Xilong Liang, Chengbing Qin, Liantuan Xiao, Dongming Sun, Jing Zhang, Zheng Han and Xiaoxi Li
A fascinating nature of two-dimensional materials is that they are atomic layer thin films that can be electrically-driven into vibration and serve as nano-electro-mechanical resonators (NEMS) with characteristic resonance frequency above 100 MHz and often with ultrahigh quality factors at low temperatures. When taking into account optical degrees of freedom, NEMS can be expanded into nano-opto-electro-mechanical systems, known as NOEMS, which is a class of hybrid solid devices designed to couple optical, electrical and mechanical degrees of freedom in nano scales, especially using functional nano-materials. To date, studies of NOEMS using van der Waals (vdW) heterostructures are very limited, although vdW materials are known for emerging phenomena such as spin, valley, and topological physics.
Lead by researchers from Shanxi University and international collaborators, a universal method has been devised to easily and robustly fabricate vdW heterostructures into an architecture that hosts opto-electro-mechanical couplings in one single device. The team demonstrated several functionalities, including nano-mechanical resonators, vacuum channel diodes, and ultra-fast thermo-radiators, using monolithically sculpted graphene/h-BN NOEMS as a platform. Optical readout of electric and magnetic field tuning of mechanical resonance in a CrOCl/graphene vdW NOEMS is further demonstrated, suggesting that the introduction of the vdW heterostructure into the NOEMS family will be of particular potential for the development of novel lab-on-a-chip systems.
In order to obtain arrays of suspended vdW heterostructures, holey h-BN was first patterned via standard lithography and plasma etching, and a dry transfer step was performed using pre-stacked few-layer vdW materials, as shown in the work-flow in Figure 1. Arrays of suspended vdW heterostructures were then monolithically sculpted at the final step of plasma etching.
“In this way, we can obtain a high yield of suspended vdW heterostructures, without the use of critical point drier”, said by the leading researcher Dr. Xiaoxi Li, “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.” It is known that transverse voltage in a suspended vdW heterostructure is important to measure such as valley Hall and quantum Hall effects (Figure 2), during optical pumping/probing, which had remained a technical challenge in suspended 2D materials. Upon a suspension height of 300 nm and a suspended area of 5 μm2, the success rate reaches 90%, as pointed out by Dr. Li.
“Thanks to the abundant library of 2D materials, we can therefore select and design all kinds of vdW heterostructures based NOEMS at our needs,” said by Dr. Tongyao Zhang, the first author of this research work, “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. Furthermore, their functionalities can be selectively modulated by patterning the 2D stacks into photonic crystals, which opens up opportunities for novel NEOMS.”
Taking h-BN/graphene heterostructure as an example, using the method described above, multi-functionalities can be established in a single suspended vdW heterostructure. First of all, the resulting devices can act as nano-mechanical resonators, with resonance at the order of 0.1 GHz and quality factor reaching 1000 at room temperature. Second, when the lower surface of the suspended vdW heterostructure is set to be graphene, the whole system can serve as a vertical vacuum channel diode. Thermionic emission of electrons can be pulled out of Joule heated graphene and collected by the bottom collector electrode, with on-off ratios reaching a level up to 105, which remained unchanged after a total ionizing dose of 3 Mrad (Si) (Figure 3). Thirdly, such a typical device can be functioning at the ultra-fast regime as thermo-radiators, with a wavelength of emitted light tunable by the depth of the suspension cavity.
The team also demonstrated an optical readout of electric and magnetic field tuning of mechanical resonance in a CrOCl/graphene vdW NOEMS. A mechanical resonance frequency shift of about 0.8 MHz was found at the magnetic phase transition in the system, exhibiting potentials for opto-mechanical detection of complex electromagnetic responses in vdW heterostructures.
“Our approach is an exciting attempt of facile and robust fabrication protocol toward suspended vdW heterostructures,” said by one of the leading researchers Prof. Chengbing Qin, “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.” The suspended vdW heterostructures thus help to explore many interesting properties in both classical and quantum regimes, holding promise in future applications of modern nano-sensors.
Journal
Light Science & Applications
Article Publication Date
1-Mar-2022