Graphene grain boundaries typically contain heptagon-pentagon pairs. The arrangement of such defects depends on whether the GB is in zig-zag or armchair direction. It further depends on the tilt-angle of the GB. In 2010, researchers from Brown University computationally predicted that as the tilt-angle increases, the grain boundary strength also increases. They showed that the weakest link in the grain boundary is at the critical bonds of the heptagon rings. As the grain boundary angle increases, the strain in these heptagon rings decreases, causing the grain-boundary to be stronger than lower-angle GBs. They proposed that, in fact, for sufficiently large angle GB, the strength of the GB is similar to pristine graphene. In 2012, it was further shown that the strength can increase or decrease, depending on the detailed arrangements of the defects. These predictions have since been supported by experimental evidences. In a 2013 study led by James Hone's group, researchers probed the elastic stiffness and strength of CVD-grown graphene by combining nano-indentation and high-resolution TEM. They found that the elastic stiffness is identical and strength is only slightly lower than those in pristine graphene. In the same year, researchers from UC Berkeley and UCLA probed bi-crystalline graphene with TEM and AFM. They found that the strength of grain-boundaries indeed tend to increase with the tilt angle.
Turbostratic graphene exhibits weak interlayer coupling, and the spacing is increased with respect to Bernal-stacked multilayer graphene. Rotational misalignment preserves the 2D electronic structure, as confirmed by Raman spectroscopy. The D peak is very weak, whereas the 2D and G peaks remain prominent. A rather peculiar feature is that the I2D/IG ratio can exceed 10. However, most importantly, the M peak, which originates from AB stacking, is absent, whereas the TS1 and TS2 modes are visible in the Raman spectrum. The material is formed through conversion of non-graphenic carbon into graphenic carbon without providing sufficient energy to allow for the reorganization through annealing of adjacent graphene layers into crystalline graphitic structures.
Geim and Novoselov initially used adhesive tape to pull graphene sheets away from graphite. Achieving single layers typically requires multiple exfoliation steps. After exfoliation the flakes are deposited on a silicon wafer. Crystallites larger than 1 mm and visible to the naked eye can be obtained.
This is where graphene starts to get reallyinteresting! Materials that conduct heat very well also conductelectricity well, because both processes transport energy using electrons. The flat, hexagonal lattice ofgraphene offers relatively little resistance to electrons, which zipthrough it quickly and easily, carrying electricity better than evensuperb conductors such as copper and almost as well assuperconductors (unlike superconductors, whichneed to be cooled to low temperatures, graphene's remarkableconductivity works even at room temperature).Scientificallyspeaking, we could say that the electrons in graphene have a longermean free path than they have in any other material (in other words,they can go further without crashing into things or otherwise beinginterrupted, which is what causes electrical resistance).What use isthis? Imagine a strong, light, relatively inexpensive material that can conductelectricity with greatly reduced energy losses: on a large scale, itcould revolutionize electricity production and distribution frompower plants; on a much smaller scale, it might spawn portablegadgets (such as cellphones) with much longer battery life.
Electrical conductivity is just about "ferrying"electricity from one place to another in a relatively crude fashion;much more interesting is manipulating the flow of electrons thatcarry electricity, which is what electronics is all about. As youmight expect from its other amazing abilities, the electronicproperties of graphene are also highly unusual. First off, theelectrons are faster and much more mobile, which opens up thepossibility of computer chips that work more quickly (and with lesspower) than the ones we use today. (In 2016, MIT researchers floated the possibility of optical graphenechips that might be a million times faster than the ones we use today.)Second, the electrons move through graphene a bitlike photons (wave-like particles of light),at speeds close enough to the speed of light (about 1 million meters per second, in fact) that they behave according to both the theories of relativity and quantum mechanics, where simple certainties are replaced by puzzling probabilities. That meanssimple bits of carbon (graphene, in other words) can be used to test aspects of those theories on the table top, instead of by usingblisteringly expensive particle accelerators or vast, powerful space telescopes.
Take a pencil and some sticky tape. Stick the tapeto the graphite, peel it away, and you'll get a layer of graphitemade up of multiple layers of carbon atoms. Repeat the process verycarefully, over and over again, and you'll (hopefully) end up withcarbon so thin that it'll contain just one layer of atoms. That'syour graphene! This rather crude method goes by the technicalname of mechanical exfoliation.An alternative method involves loading up a super-preciseatomic force microscope witha piece of graphite and then rubbing it very precisely on somethingso that single layers of graphene flake off, a bit like graphitefrom a pencil lead only one layer at a time. Techniques like this arefiddly and intricate and explain why graphene is currently one of themost expensive materials on the planet!
In theory, we could use graphene to makeballistic transistors that store information or switchon and off at super-high speeds by manipulating single electrons. In much the same way,graphene could revolutionize other areas of technology constrainedby conventional materials. For example, it could spawn lighterand stronger airplanes (by replacing composite materials or metalalloys), cost-competitive and more efficient solar panels (replacing silicon again),more energy-efficient power transmission equipment (in place of superconductors), andsupercapacitors withthinner plates that can be charged in seconds and store more energy in a smaller space thanhas ever previously been possible (replacing ordinary, chemicalbatteries entirely). Companies such as Samsung, Nokia, and IBM arealready developing graphene-based replacements for such things astouchscreens, transistors, and flash memories, though the work is ata very early stage.
Based on the natural periodicity of both the structure and CEB, we present a novel physical mechanism of generation of THz radiation. The mechanism involves two processes, (i) excitation of SPPs in circular cylindrical graphene structures with a CEB inside the light cone of the dielectrics and (ii) immediate transformation of the excited SPPs into Cherenkov THz radiation. Our theoretical analysis and numerical simulation show that, based on this mechanism, the room temperature, coherent, tunable THz radiation sources with high power density, can be developed. Moreover, the SPPs are propagating along the cyclotron trajectory together with the electron beam, maintaining synchronization between SPPs and CEB in both the angular velocity and longitudinal phase-velocity. This synchronization assures that SPPs can gain energy from the electron beam continuously to compensate the loss due to the radiation and decay. Both monolayer and double-layer graphene structures are proposed and studied. In case of the double-layer structure, two-color THz radiation can be achieved.
Graphene is a promising material for photodetectors compared to conventional semiconductors due to ultrahigh mobility, making it suitable for high-speed communications , , , . Single-atomic-layer graphene is stable, low cost, easy to fabricate, and has high internal quantum efficiency  when used in the NIR and MIR regions for optoelectronic applications. However, single-layer graphene is an inherently weak light absorber, which originates from its short interaction length , , , and the narrow effective area of lateral graphene p-n junctions also limits the efficiency of photocarrier extraction , , . In other words, the applications of grapheme-based photodetectors are limited by the lower external quantum efficiency and photoresponsivity in comparison to traditional photodetectors .
Recently, another family of 2D materials with far better optical properties has emerged. Single-layer TMDCs , such as MoS2 and WSe2, have been intensely researched. The direct bandgap of monolayer TMDCs leads to efficient light emission covering the energy range from below 1 eV to well above 2.5 eV  and beyond, making them promising for a wide range of optoelectronic devices. Bandgap tunability with the layer thickness is another important property of this material. The bandgaps of trilayer, bilayer, and monolayer MoS2, determined by photoluminescence, change from 1.35 to 1.65 and 1.8 eV , respectively. The bandgap of other typical TMDCs, such as WSe2, MoSe2 , WS2 , and GaTe , also increases with decreasing layer thickness, due to the quantum confinement of carriers in the direction normal to the 2D plane. 2D TMDC photodetectors exhibit decent responsivity from the IR to the near UV range. The first reported monolayer MoS2 photodetector  exhibited a responsivity of 7.5 mA/W in the visible range. Choi et al.  demonstrated a responsivity of ~100 mA/W by using a multilayer MoS2 photodetector. However, the speed of photoresponse is relatively slow (ranging from microseconds to seconds) owing to the trapping of photocarriers. It is also noticeable that most of the high-sensitivity TMDC-based photodetectors reported above are for visible and NIR application. However, the recently re-discovered few-layer black phosphorus (bP) is an interesting material for photodetection for IR and far infrared (FIR), especially due to its intermediate bandgap between graphene (i.e. zero bandgap) and TMDCs. The study by Youngblood et al.  shows an intrinsic responsivity of up to 135 mA/W and 657 mA/W in 11.5-nm- and 100-nm-thick devices, respectively, at room temperature and at a frequency of 3 GHz. The few-layer bP is embedded within an on-chip waveguide structure with a few-layer graphene top gate, allowing for optimal interaction with light and tunability of the carrier density. The bP-based device presented in  stands out from the rest by showing comparable performance to both a commercial silicon photodiode and a graphene-based detector. Despite the large amount of funding and research invested on 2D materials, there is currently very limited set of 2D material covering the IR region of the spectrum . This might represent an important future research area, considering the strong need for conformal and thin IR detectors. 2b1af7f3a8