Optical reflectivity contrast provides a simple, fast and noninvasive method for characterization of few monolayer samples of two-dimensional materials. Here we apply this technique to measure the thickness of thin flakes of hexagonal Boron Nitride (hBN), which is a material of increasing interest in nanodevice fabrication. The optical contrast shows a strong negative peak at short wavelengths and zero contrast at a thickness dependent wavelength. The optical contrast varies linearly for 1-80 layers of hBN, which permits easy calibration of thickness. We demonstrate the applicability of this quick characterization method by comparing atomic force microscopy and optical contrast results.Hexagonal Boron Nitride (hBN) has a planar hexagonal structure similar to graphite and has proven to be an excellent substrate for graphene based electronic and opto-electronic devices. It has been shown that graphene devices on hBN substrates show enhanced performance like increased carrier mobility and reduced charge fluctuations 1-3 . Graphene sheets conform to atomically flat hBN resulting in reduced roughness and charge puddle formation as compared to other common substrates such as Si/SiO 2 2,3 . Thin hBN flakes have also proven to be an excellent dielectric or tunnel barrier for device applications 4-7 and can modify graphene's band structure 8 . A large direct bandgap makes hBN attractive for compact UV laser applications 9 . Interestingly, the success of hBN in graphene electronics is now also being mirrored in the development of other two dimensional materials such as transition metal dichalcogenides (TMD) (e.g. MoS 2 , MoSe 2 , WS 2 etc.) devices, where hBN substrates have led to 10 times better photoluminescence quantum yields than Si/SiO 2 . 10 It is anticipated that hBN will be an essential constituent for future graphene and TMD heterostructure devices in many roles ranging from a tunnel barrier to a gate dielectric. Hence, it is very important to have methods for quick, economical and non-invasive characterization of hBN flakes, specifically, the exact number of hBN monolayers and its flatness over the size of the device. In that regards optical reflection microscopy has proven to be a highly useful tool. Optical contrast measurement have been used to identify mono and few layered graphene on various substrates 11-13 . Identification of monolayer and bilayer hBN has also been reported using reflectivity contrast 14 . In this paper, we characterize hBN flakes deposited on a SiO 2 /Si substrate. We establish parameters which can enable quick identification for hBN flakes varying from a few to 100 layers. We also show that this approach is sensitive to optical thickness changes as small as 1-2 layers allowing the identification of steps in flakes which appear flat under white light illumination.The sample geometry is shown schematically in the inset of figure 1. Few layer hBN flakes were prepared by FIG. 1. The optical contrast of hBN on SiO2/Si substrate as a function of the wavelength of light. Different curves corre...

We visualize ATP-driven domain dynamics of individual SecA molecules in a near-native setting using atomic force microscopy.

Escherichia coli exports proteins via a translocase comprising SecA and the translocon, SecYEG. Structural changes of active translocases underlie general secretory system function, yet directly visualizing dynamics has been challenging. We imaged active translocases in lipid bilayers as a function of precursor protein species, nucleotide species, and stage of translocation using atomic force microscopy (AFM). Starting from nearly identical initial states, SecA more readily dissociated from SecYEG when engaged with the precursor of outer membrane protein A as compared to the precursor of galactose-binding protein. For the SecA that remained bound to the translocon, the quaternary structure varied with nucleotide, populating SecA2 primarily with adenosine diphosphate (ADP) and adenosine triphosphate, and the SecA monomer with the transition state analog ADP-AlF3. Conformations of translocases exhibited precursor-dependent differences on the AFM imaging time scale. The data, acquired under near-native conditions, suggest that the translocation process varies with precursor species.

The electronic band structure of twisted bilayer graphene develops van Hove singularities whose energy depends on the twist angle between the two layers. Using Raman spectroscopy, we monitor the evolution of the electronic band structure upon doping using the G peak area which is enhanced when the laser photon energy is resonant with the energy separation of the van Hove singularities. Upon charge doping, the Raman G peak area initially increases for twist angles larger than a critical angle and decreases for smaller angles. To explain this behavior with twist angle, the energy separation of the van Hove singularities must decrease with increasing charge density demonstrating the ability to modify the electronic and optical properties of twisted bilayer graphene with doping.

Raman spectroscopy, a fast and nondestructive imaging method, can be used to monitor the doping level in graphene devices. We fabricated chemical vapor deposition (CVD) grown graphene on atomically flat hexagonal boron nitride (hBN) flakes and SiO2 substrates. We compared their Raman response as a function of charge carrier density using an ion gel as a top gate. The G peak position, the 2D peak position, the 2D peak width and the ratio of the 2D peak area to the G peak area show a dependence on carrier density that differs for hBN compared to SiO2. Histograms of two-dimensional mapping are used to compare the fluctuations in the Raman peak properties between the two substrates. The hBN substrate has been found to produce fewer fluctuations at the same charge density owing to its atomically flat surface and reduced charged impurities.