Gate dependent Raman spectroscopy of graphene on hexagonal boron nitride

Design and fabrication of a large-range graphene/hexagonal boron nitride heterostructure based pressure sensor with poly(methyl methacrylate) substrate

Aiming at overpressure measurement, this paper presents a large-range graphene/hexagonal boron nitride (h-BN) heterostructure-based pressure sensor with a poly(methyl methacrylate) (PMMA) substrate. Graphene and h-BN are chosen as sensitive materials because they both have large Young's modulus, high intrinsic strength, high natural frequency, and atomic thickness at the same time. These characteristics provide favorable conditions for the application of the sensor in the high pressure and high frequency dynamic environment. Moreover, the photoresist-assisted transfer technology is proposed for transferring graphene from the growth substrate to the PMMA substrate and the lift-off method with exposure and development is developed to achieve metal patterning on the PMMA substrate. The sensor characterization results suggest that the graphene and h-BN films have good transfer qualities and the heterojunction possesses excellent electrical performance. The static pressure loading experiments confirm that the sensor has a pressure range of up to 85 MPa and its piezoresistive coefficient is 0.7 GPa^-1, which indicates that the designed sensor is suitable for overpressure fields. This study provides a novel method for determining overpressure and lays a foundation for the fabrication of graphene-based electronic devices with an organic substrate.

Near-field probing of dielectric screening by hexagonal boron nitride in graphene integrated on silicon photonics

Hexagonal boron nitride (hBN) is one of the most suitable 2D materials for supporting graphene in electronic devices, and it plays a fundamental role in screening out the effect of charge impurities in graphene in contrast to inhomogeneous supports such as silicon dioxide (SiO₂). Although many interesting surface science techniques such as scanning tunneling microscopy (STM) revealed dielectric screening by hBN and emergent physical phenomena were observed, STM is only appropriate for graphene electronics. In this paper, we demonstrate the dielectric screening by hBN in graphene integrated on a silicon photonic waveguide from the perspective of a near-field scanning optical microscopy (NSOM) and Raman spectroscopy. We found shifts in the Raman spectra and about three times lower slope decrease in the measured electric near-field amplitude for graphene on hBN relative to that for graphene on SiO₂. Based on finite-difference time-domain simulations, we confirm lower electric field slope and scattering rate in graphene on hBN, which implies dielectric screening, in agreement with the NSOM signal. Graphene on hBN integrated on silicon photonics can pave the way for high-performance hybrid graphene photonics.

Sensitive Raman Probe of Electronic Interactions between Monolayer Graphene and Substrate under Electrochemical Potential Control

In situ electrochemical Raman spectroscopic measurements of defect-free monolayer graphene on various substrates were performed under electrochemical potential control. The G and 2D Raman band wavenumbers (ω_G, ω_2D) of graphene were found to depend upon the electrochemical potential, i.e., the charge density of graphene. The values of ω_G and ω_2D also varied depending on the choice of substrates. On metal substrates where graphene was synthesized by chemical vapor deposition, a strong blue shift of ω_2D was induced, which could not account for the strain and charge doping. We attributed the blue shift of ω_2D to a change in the electronic properties of graphene induced by distinct electronic interactions with the metal substrates. To explain the unique characteristics in the Raman spectrum of graphene on various substrates, a novel mechanism is proposed considering reduction of the Fermi velocity in graphene owing to dielectric screening from the metal substrates.

Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper

Graphene research has prospered impressively in the past few years, and promising applications such as high-frequency transistors, magnetic field sensors, and flexible optoelectronics are just waiting for a scalable and cost-efficient fabrication technology to produce high-mobility graphene. Although significant progress has been made in chemical vapor deposition (CVD) and epitaxial growth of graphene, the carrier mobility obtained with these techniques is still significantly lower than what is achieved using exfoliated graphene. We show that the quality of CVD-grown graphene depends critically on the used transfer process, and we report on an advanced transfer technique that allows both reusing the copper substrate of the CVD growth and making devices with mobilities as high as 350,000 cm(2) V(-1) s(-1), thus rivaling exfoliated graphene.

Graphene on hexagonal boron nitride

The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices as it is insulating, atomically flat and provides a clean charge environment for the graphene. Additionally, the interaction between graphene and boron nitride provides a path for the study of new physical phenomena not present in bare graphene devices. This review focuses on recent advancements in the study of graphene on hexagonal boron nitride devices from the perspective of scanning tunneling microscopy with highlights of some important results from electrical transport measurements.