Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> Finite graphite systems having a zigzag edge exhibit a special edge state. The corresponding energy bands are almost flat at the Fermi level and thereby give a sharp peak in the density of states. The charge density in the edge state is strongly localized on the zigzag edge sites. No such localized state appears in graphite systems having an armchair edge. By utilizing the graphene ribbon model, we discuss the effect of the system size and edge shape on the special edge state. By varying the width of the graphene ribbons, we find that the nanometer size effect is crucial for determining the relative importance of the edge state. We also have extended the graphene ribbon to have edges of a general shape, which is defined as a mixture of zigzag and armchair sites. Examining the relative importance of the edge state for graphene ribbons with general edges, we find that a non-negligible edge state survives even in graphene ribbons with less developed zigzag edges. We demonstrate that such an edge shape with three or four zigzag sites per sequence is sufficient to show an edge state, when the system size is on a nanometer scale. The special characteristics of the edge state play a large role in determining the density of states near the Fermi level for graphite networks on a nanometer scale. <s> BIB001 </s> Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> We study the existence and topological stability of Fermi points in a graphene layer and stacks with many layers. We show that the discrete symmetries (space-time inversion) stabilize the Fermi points in monolayer, bilayer, and multilayer graphenes with orthorhombic stacking. The bands near $k=0$ and $ϵ=0$ in multilayers with the Bernal stacking depend on the parity of the number of layers, and Fermi points are unstable when the number of layers is odd. The low-energy changes in the electronic structure induced by commensurate perturbations which mix the two Dirac points are also investigated. <s> BIB002 </s> Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> We study the electronic states of narrow graphene ribbons (``nanoribbons'') with zigzag and armchair edges. The finite width of these systems breaks the spectrum into an infinite set of bands, which we demonstrate can be quantitatively understood using the Dirac equation with appropriate boundary conditions. For the zigzag nanoribbon we demonstrate that the boundary condition allows a particlelike and a holelike band with evanescent wave functions confined to the surfaces, which continuously turn into the well-known zero energy surface states as the width gets large. For armchair edges, we show that the boundary condition leads to admixing of valley states, and the band structure is metallic when the width of the sample in lattice constant units has the form $3M+1$, with $M$ an integer, and insulating otherwise. A comparison of the wave functions and energies from tight-binding calculations and solutions of the Dirac equations yields quantitative agreement for all but the narrowest ribbons. <s> BIB003 </s> Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> Graphene has shown great application potential as the hostmaterial for next-generation electronic devices. However, despite itsintriguing properties, one of the biggest hurdles for graphene to beuseful as an electronic material is the lack of an energy gap in itselectronic spectra. This, for example, prevents the use of graphene inmaking transistors. Although several proposals have been made to open agap in graphene's electronic spectra, they all require complexengineering of the graphene layer. Here, we show that when graphene isepitaxially grown on SiC substrate, a gap of ~;0.26 eV is produced. Thisgap decreases as the sample thickness increases and eventually approacheszero when the number of layers exceeds four. We propose that the originof this gap is the breaking of sublattice symmetry owing to thegraphene-substrate interaction. We believe that our results highlight apromising direction for band gap engineering of graphene. <s> BIB004 </s> Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> On the basis of first-principles calculations, we report that a novel interfacial atomic structure occurs between graphene and the surface of silicon carbide, destroying the Dirac point of graphene and opening a substantial energy gap there. In the calculated atomic structures, a quasiperiodic 6x6 domain pattern emerges out of a larger commensurate 6 sqrt [3] x 6 sqrt [3]R30 degrees periodic interfacial reconstruction, resolving a long standing experimental controversy on the periodicity of the interfacial superstructures. Our theoretical energy spectrum shows a gap and midgap states at the Dirac point of graphene, which are in excellent agreement with the recently observed anomalous angle-resolved photoemission spectra. Beyond solving unexplained issues in epitaxial graphene, our atomistic study may provide a way to engineer the energy gaps of graphene on substrates. <s> BIB005 </s> Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> We studied the effect of quantum confinement on the size of the band gap in single layer epitaxial graphene. Samples with different graphene terrace sizes are studied by using low energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES). The direct correlation between the terrace size extracted from LEEM and the gap size extracted from ARPES shows that quantum confinement alone cannot account for the large gap observed in epitaxial graphene samples. <s> BIB006 </s> Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> The electronic properties of epitaxial graphene grown on SiC have shown its potential as a viable candidate for post-CMOS electronics. However, progress in this field requires a detailed understanding of both the structure and growth of epitaxial graphene. To that end, this review will focus on the current state of epitaxial graphene research as it relates to the structure of graphene grown on SiC. We pay particular attention to the similarity and differences between graphene growth on the two polar faces, (0001) and , of hexagonal SiC. Growth techniques, subsequent morphology and the structure of the graphene/SiC interface and graphene stacking order are reviewed and discussed. Where possible the relationship between film morphology and electronic properties will also be reviewed. <s> BIB007 </s> Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> This letter presents a simple method for transferring epitaxial sheets of graphene on silicon carbide to other substrates. The graphene was grown on the (0001) face of 6H-SiC by thermal annealing at 1550 °C in a hydrogen atmosphere. Transfer was accomplished using a peeling process with a bilayer film of gold/polyimide, to yield graphene with square millimeters of coverage on the target substrate. Raman spectroscopy provided evidence that the transferred material is single layer. Back gated field-effect transistors fabricated on oxidized silicon substrates with Cr/Au as source-drain electrodes exhibited ambipolar characteristics with hole mobilities of ∼100 cm2/V-s, and negligible influence of resistance at the contacts. <s> BIB008 </s> Graphene-Based Materials for Biosensors: A Review <s> Epitaxial Growth on SiC <s> In this paper, we carried out detailed electrochemical studies of epitaxial graphene (EG) using inner-sphere and outer-sphere redox mediators. The EG sample was anodized systematically to investigate the effect of edge plane defects on the heterogeneous charge transfer kinetics and capacitive noise. We found that anodized EG, consisting of oxygen-related defects, is a superior biosensing platform for the detection of nucleic acids, uric acids (UA), dopamine (DA), and ascorbic acids (AA). Mixtures of nucleic acids (A, T, C, G) or biomolecules (AA, UA, DA) can be resolved as individual peaks using differential pulse voltammetry. In fact, an anodized EG voltammetric sensor can realize the simultaneous detection of all four DNA bases in double stranded DNA (dsDNA) without a prehydrolysis step, and it can also differentiate single stranded DNA from dsDNA. Our results show that graphene with high edge plane defects, as opposed to pristine graphene, is the choice platform in high resolution electrochemical sensing. <s> BIB009
A different method starts with silicon carbide (SiC) heated up to 1250-1450 • C under ultrahigh vacuum (UHV) as seen in Figure 3d . The silicon ions will then sublimate, leaving behind the carbon atoms on the surface of SiC. Upon cooling under optimized conditions, the carbon atoms will self-organize into a honeycomb structure. The epitaxial graphene (EG) on SiC can then be transferred onto a desired substrate as free-standing graphene. For example, EG achieves the transfer by using a thin gold layer and polyimide (PI) to peel-off from SiC to a SiO 2 /Si wafer. The resulting graphene layer however, has high amounts of defects with very low mobility of 100 cm 2 /Vs BIB008 . This method has difficulty in controlling the number of layers formed in the temperature control and the presence of defects or disorder in the hexagonal structure. More importantly, the electronic properties of EG have been characterized and indicate an opening of a band gap about 260 meV where the Fermi level shifts to 400 meV BIB004  . The size of the band gap also depends on the graphene thickness. A number of researchers have studied intensively the energy bandgap formation, growth mechanism, and the symmetry breaking at the graphene-SiC interface BIB006 BIB007 . There are two possible origins to describe the gap opening phenomenon between the two bands. One is to hybridize the electronic states at the Dirac points, which requires the translation symmetry to be broken BIB002 . The other one is to break the equivalence between atomic sublattices in graphene, which does not require any translation symmetry breaking BIB001 BIB003 . The opening of the gap is believed to be due to the breaking of the symmetry between the K and K' points in the Brillouin zone by the graphene layer and the SiC substrate interaction. The EG appeared on the surface of SiC substrate which has a quasiperiodic 6 × 6 domain pattern and the atomic pattern of a 6 √ 3 × 6 √ 3R30 • periodicity BIB005 . The utilization of EG on SiC is promising for making an electroanalytical platform in a wide range of biosensors which is comparable to carbon nanotubes, boron-doped diamond, and glassy carbon. EG-based biosensors are demonstrated to resolve all four nucleic acid bases and are able to distinguish dopamine, ascorbic acid, and uric acid at physiological pH in which the electrochemical performances contribute to the presence of edge and plane defects BIB009 . Although the cost of SiC wafer remains a limiting factor for industrial fabrication, EG offers easy integration into existing electronic procedures and importantly, the energy band gap enables an electronic off-state used in graphene transistors and sensor devices.