Suspended Multilayer Graphene Research Articles

Sealing Graphene Nanodrums.

Despite theoretical predictions that graphene should be impermeable to all gases, practical experiments on sealed graphene nanodrums show small leak rates. Thus far, the exact mechanism for this permeation has remained unclear, because different potential leakage pathways have not been studied separately. Here, we demonstrate a sealing method that consists of depositing SiO2 across the edge of suspended multilayer graphene flakes using electron beam-induced deposition. By sealing, leakage along the graphene–SiO2 interface is blocked, which is observed to result in a reduction in permeation rate by a factor of 104. The experiments thus demonstrate that gas flow along the graphene–SiO2 interface tends to dominate the leak rate in unsealed graphene nanodrums. Moreover, the presented sealing method enables the study of intrinsic gas leakage through graphene membranes and can enable hermetic graphene membranes for pressure sensing applications.

Very large suspended graphene as an efficient electron-transparent gate electrode

Although a focused electron beam is required in field emission-based various applications, the electron beam is generally diverged in a conventional triode because the gate has a circular aperture through which the electrons are accelerated to reach the anode. To prevent the beam divergence, a suspended multi-layer graphene (MLG) was fabricated on a very large gate aperture to be used as an electron-transparent gate in a field emission triode. The suspended MLG was observed to have several micro-openings with an average diameter of ∼5 μm formed during a transfer. It was observed that the MLG with micro-openings was likely to act as an efficient transparent gate for electrons even with large number of graphene layers, exhibiting a high transmittance of ∼90%. The solid angles of the beams obtained using conventional gate aperture and suspended MLG gate were 0.225 and 0.074 sr, respectively, indicating that the suspended MLG resulted in a focused electron beam. Simulations confirm that the suspended MLG with micro-openings suppresses the beam divergence in a given triode configuration because the MLG results in the flattened electric field profiles between gate and cathode whereas a distorted potential distribution causing beam divergence occurs around the aperture in a conventional triode.

Periodic oscillation in the reflection and photoluminescence spectra of suspended two-dimensional crystal flakes

Suspended two-dimensional (2D) materials have been widely used to improve the device performances in comparison with the case of supported 2D materials. To realize such a purpose, 2D materials are mainly suspended on the holes of substrates, which are usually used to support 2D materials. The holes beneath the 2D materials are usually full of air. The air layer with the thickness identical to the hole depth will affect the spectral features of the reflection and photoluminescence spectra of suspended 2D materials because there exist multiple optical interferences in the air/2D-flakes/air/Si multilayer structures. However, it is not clear that how the spectral features depend on the hole depth. In this paper, the reflection spectra of suspended multilayer graphene and MoS2flakes as well as the photoluminescence spectra of suspended multilayer MoS2flakes are systematically studied. The reflection spectra of suspended multilayer graphene flakes exhibit obvious oscillations, showing the optical characteristic with periodic oscillations in wavenumber. The oscillation period decreases with increasing the hole depth (or the thickness of the air layer), but is independent of the thickness of suspended graphene flakes. This can be well explained by the model based on multiple optical interferences in the air/graphenes/air/Si multilayer structures, which have been successfully utilized to understand the Raman intensity of ultrathin 2D flakes and substrate beneath the ultrathin 2D flakes dependent on the thickness of 2D flakes, the thickness of SiO2 layer, the laser wavelength and the numerical aperture of objective. The theoretical simulation shows that the oscillation is obviously observable only when the hole depth reaches up to the value on the order of microns. For suspended multilayer MoS2flakes, the reflection and photoluminescence spectra show similar periodic oscillations in wavenumber and the oscillation period is also dependent on the hole depth. The hole depth is measured by the surface profiler. It is found that the calculated oscillation period based on the measured hole depth and multiple optical interference model is usually larger than the experimental one, which is attributed to the existence of the dielectric layer in the holes. The dielectric layer may be the residues after the hole etching process, which have much smaller refractive indexes than substrates and 2D flakes. This results in an increase of the effective hole depth, which becomes larger than the one measured by the surface profiler. The observation of oscillation period in the reflection and photoluminescence spectra of different flakes of 2D materials demonstrates that the periodic oscillation is a general optical characteristic for optical spectra of suspended 2D materials. In principle, the electroluminescence spectra of suspended 2D materials may also exhibit similar periodic oscillations in wavenumber. These findings may be helpful for understanding the optical spectra of various suspended 2D materials and monitoring the existence of the residues in the holes of substrate after the etching process.

Fabrication process and characterization of suspended graphene membranes for RF NEMS capacitive switches

The fabrication of a Nanoelectromechanical System (NEMS) capacitive switch consisting of a suspended multilayer graphene membrane, doubly clamped on a coplanar waveguide (CPW) is presented and its RF performance is evaluated. The scattering parameters are measured and compared with theory in order to assess rigorously the advantages of using multilayer graphene as a membrane for the switch. Our results show that while, in theory, graphene remains a promising candidate for NEMS applications, significant efforts are needed for improving the fabrication process of suspended graphene membranes and the quality and reproducibility of reported devices.

Fiber-optic ferrule-top nanomechanical resonator with multilayer graphene film.

Compact ferrule-top nanomechanical resonators with multilayer graphene (MLG) diaphragms as vibrating elements are demonstrated. The resonators comprise a suspended MLG film supported by a ceramic ferrule with a bore diameter of ∼125 μm. The mechanical resonance of the graphene film is excited and detected by an all-fiber optical interrogation system. Based on a beam-shape graphene mechanical resonator, a force sensitivity of ∼3.8 fN/Hz1/2 was theoretically predicted. The integration of nanomechanical graphene film with optical fiber simplifies the excitation and interrogation of the resonator and would allow the development of practical fiber-optic sensors for force, mass, and pressure measurements.

Nonlocal laser annealing to improve thermal contacts between multi-layer graphene and metals

The accuracy of thermal conductivity measurements by the micro-Raman technique for suspended multi-layer graphene flakes has been shown to depend critically on the quality of the thermal contacts between the flakes and the metal electrodes used as the heat sink. The quality of the contacts can be improved by nonlocal laser annealing at increased power. The improvement of the thermal contacts to initially rough metal electrodes is attributed to local melting of the metal surface under laser heating, and increased area of real metal–graphene contact. Improvement of the thermal contacts between multi-layer graphene and a silicon oxide surface was also observed, with more efficient heat transfer from graphene as compared with the graphene–metal case.

Low contact resistivity and strain in suspended multilayer graphene

Method to prepare suspended multilayer graphene (MLG) flakes and to form highly conductive (contact resistivity of ∼0.1 kΩ μm2) and tight mechanical connection between MLG and metal electrodes is described. MLG flakes prepared from natural graphite were precisely deposited over tungsten electrodes using dielectrophoresis, followed by high-temperature thermal annealing in high-vacuum. Considerable strain induced in the suspended part of flakes was revealed by Raman imaging.

Hot electron transport in suspended multilayer graphene

We study hot electron transport in short-channel suspended multilayer graphene devices created by a distinct experimental approach. For devices with semitransparent contact barriers, a dip of differential conductance $(dI/dV)$ has been observed at source-drain bias ${V}_{d}=0$, along with anomalies at higher ${V}_{d}$ likely induced by optical-phonon scattering. For devices with low-contact barriers, only the $dI/dV$ dip at ${V}_{d}=0$ is observed, and we find a well-fit logarithmic dependence of $dI/dV$ on both the bias ${V}_{d}$ and the temperature $T$. The logarithmic ${V}_{d}$ dependence is explained with the hot electron effect and the logarithmic $T$ dependence could be attributed to the weak localization in two dimensions.

Electron beam nanosculpting of suspended graphene sheets

We demonstrate high-resolution modification of suspended multilayer graphene sheets by controlled exposure to the focused electron beam of a transmission electron microscope. We show that this technique can be used to realize, on time scales of a few seconds, a variety of features, including nanometer-scale pores, slits, and gaps that are stable and do not evolve over time. Despite the extreme thinness of the suspended graphene sheets, extensive removal of material to produce the desired feature geometries is found not to introduce long-range distortion of the suspended sheet structure.