Band Gap In Bilayer Graphene Research Articles

Molecular Doping and Band-Gap Opening of Bilayer Graphene

The ability to induce an energy band gap in bilayer graphene is an important development in graphene science and opens up potential applications in electronics and photonics. Here we report the emergence of permanent electronic and optical band gaps in bilayer graphene upon adsorption of π electron containing molecules. Adsorption of n- or p-type dopant molecules on one layer results in an asymmetric charge distribution between the top and bottom layers and in the formation of an energy gap. The resultant band gap scales linearly with induced carrier density though a slight asymmetry is found between n-type dopants, where the band gap varies as 47 meV/10(13) cm(-2), and p-type dopants where it varies as 40 meV/10(13) cm(-2). Decamethylcobaltocene (DMC, n-type) and 3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ, p-type) are found to be the best molecules at inducing the largest electronic band gaps up to 0.15 eV. Optical adsorption transitions in the 2.8-4 μm region of the spectrum can result between states that are not Pauli blocked. Comparison is made between the band gaps calculated from adsorbate-induced electric fields and from average displacement fields found in dual gate bilayer graphene devices. A key advantage of using molecular adsorption with π electron containing molecules is that the high binding energy can induce a permanent band gap and open up possible uses of bilayer graphene in mid-infrared photonic or electronic device applications.

Tuning the band gap of bilayer graphene by ion implantation: Insight from computational studies

By employing molecular dynamics (MD) simulations based on empirical potentials and density functional theory, we demonstrate that the electronic properties of bilayer graphene could be tailored by means of low-energy ion irradiations. We first performed MD simulations to investigate the doping and intercalation effect in bilayer graphene induced by low-energy B and N bombardment. Our simulation shows that there are two maximal probabilities for perfect substitution of a carbon atom with incident B or N, corresponding to the two layers. The highest substitutional probability is observed for N irradiation which is 38$%$ at 70 eV in the upper layer and 33$%$ at 110 eV in the lower layer. We have calculated the energy bands for all the atomic configurations that appear after the bombardment of B and N and show that the band gap of bilayer graphene can be widely tuned via the incorporation of B and N into the bilayer graphene. The maximal band gap is found to be 392.1 meV when the B implants into a graphene layer with the knocked C forms a C-C dumbbell defect in another layer. We also investigate the probability of Au intercalated into the bilayer graphene and show that up to 93$%$ of incident Au can be trapped between the two layers when the incident energy is close to 90 eV, which gives rise to the $n$-type doping of graphene. The present results demonstrate that ion irradiation is an effective route to manipulate the structure of bilayer graphene and thus provide a way for controllable modification of its electronic properties for a variety of future nanoelectronic applications.

Tunable band gap in half-fluorinated bilayer graphene under biaxial strains

Opening and tuning of the band gap of bilayer graphene (BLG) is of particular importance for its utilization in nanoelectronics. We presented here the electronic structures of two types of stoichiometrically half-fluorinated BLGs (i.e. C2Fs) as well as those under biaxial compressive and tensile strains. Our results reveal that both C2Fs are semiconductor with large direct band gaps in their unstrained configurations. Under biaxial compressive strains, the band gaps of both C2Fs can be reduced. However, by applying biaxial tensile strains, both C2Fs undergo a direct-to-indirect band gap transition. Electronic nature of the strain-tuned band gaps has been discussed.

Opening and tuning of band gap by the formation of diamond superlattices in twisted bilayer graphene

We report results of first-principles density functional theory calculations, which introduce a new class of carbon nanostructures formed due to creation of covalent interlayer C-C bonds in twisted bilayer graphene (TBG). This interlayer bonding becomes possible by hydrogenation of the graphene layers according to certain hydrogenation patterns. The resulting relaxed configurations consist of two-dimensional (2D) superlattices of diamondlike nanocrystals embedded within the graphene layers, with the same periodicity as that of the Moir\'e pattern corresponding to the rotational layer stacking in TBG. The 2D diamond nanodomains resemble the cubic or the hexagonal diamond phase. The detailed structure of these superlattice configurations is determined by parameters that include the twist angle, ranging from 0\ifmmode^\circ\else\textdegree\fi{} to \ensuremath{\sim}15\ifmmode^\circ\else\textdegree\fi{}, and the number of interlayer C-C bonds formed per unit cell of the superlattice. We demonstrate that formation of such interlayer-bonded finite domains causes the opening of a band gap in the electronic band structure of TBG, which depends on the density and spatial distribution of interlayer C-C bonds. We have predicted band gaps as wide as 1.2 eV and found that the band gap increases monotonically with increasing size of the embedded diamond nanodomain in the unit cell of the superlattice. Such nanostructure formation constitutes a promising approach for opening a precisely tunable band gap in bilayer graphene.

Quantum transport in double-gated graphene devices

Double-gated graphene devices provide an important platform for understanding electrical and optical properties of graphene. Here we present transport measurements of single layer, bilayer and trilayer graphene devices with suspended top gates. In zero magnetic fields, we observe formation of pnp junctions with tunable polarity and charge densities, as well as a tunable band gap in bilayer graphene and a tunable band overlap in trilayer graphene. In high magnetic fields, the devices’ conductance are quantized at integer and fractional values of conductance quantum, and the data are in good agreement with a model based on edge state equilibration at pn interfaces.

Electric-field-induced band gap of bilayer graphene in ionic liquid

Ionic liquid-gated graphene field-effect-transistors (G-FETs) were fabricated to generate a band gap in bilayer graphene. The transfer characteristics of the G-FETs revealed that the transconductance when using the ionic-liquid gate was significantly higher than that when using the back gate, because an electrical double layer formed in the ionic liquid with 200-fold the capacitance of a 300-nm-thick SiO2 layer. The results indicate that the ionic-liquid-gate structure enables application of an effective electric field. Moreover, an increase in the resistance of the bilayer graphene was clearly observed as the magnitude of the electric-field intensity was increased, owing to the creation of the band gap. From measurements of electrical characteristics as a function of temperature, a band gap of 235 meV was created in bilayer graphene at an ionic-liquid-gate voltage of −3.0 V.

Bilayer graphene on h-BN substrate: investigating the breakdown voltage and tuning the bandgap by electric field

By performing density functional theory calculations we show that it is possible to make the electronic bandgap in bilayer graphene supported on hexagonal boron nitride (h-BN) substrates tunable. We also show that, under applied electric fields, it is possible to insert states from h-BN into the bandgap, which generate a conduction channel through the substrate making the system metallic. In addition, we verify that the breakdown voltage strongly depends on the number of h-BN layers. We also show that both the breakdown voltage and the bandgap tuning are independent of the h-BN stacking order.

Opening an Electrical Band Gap of Bilayer Graphene with Molecular Doping

The opening of an electrical band gap in graphene is crucial for its application for logic circuits. Recent studies have shown that an energy gap in Bernal-stacked bilayer graphene can be generated by applying an electric displacement field. Molecular doping has also been proposed to open the electrical gap of bilayer graphene by breaking either in-plane symmetry or inversion symmetry; however, no direct observation of an electrical gap has been reported. Here we discover that the organic molecule triazine is able to form a uniform thin coating on the top surface of a bilayer graphene, which efficiently blocks the accessible doping sites and prevents ambient p-doping on the top layer. The charge distribution asymmetry between the top and bottom layers can then be enhanced simply by increasing the p-doping from oxygen/moisture to the bottom layer. The on/off current ratio for a bottom-gated bilayer transistor operated in ambient condition is improved by at least 1 order of magnitude. The estimated electrical band gap is up to ∼111 meV at room temperature. The observed electrical band gap dependence on the hole-carrier density increase agrees well with the recent density-functional theory calculations. This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.

Electron transport and excitations in graphene

Graphene is a two-dimensional, single atomic layer carbon structure with many unique physical and chemical properties. In this article, I will focus on electron transport through graphene, both incoherent and coherent, the control of the current through external electric fields, and the realization of high frequency transistors based on graphene. I will also discuss briefly graphene.’s optical properties and how they can be used to create wide wavelength range, ultra-fast photodetectors. Finally, I will present results on the field-induced opening of a tunable band-gap in bi-layer graphene.

Gate-tunable bandgap in bilayer graphene

The tight-binding model of bilayer graphene is used to find the gap between the conduction and valence bands, as a function of both the gate voltage and as the doping by donors or acceptors. The total Hartree energy is minimized and the equation for the gap is obtained. This equation for the ratio of the gap to the chemical potential is determined only by the screening constant. Thus the gap is strictly proportional to the gate voltage or the carrier concentration in the absence of donors or acceptors. In the opposite case, where the donors or acceptors are present, the gap demonstrates the asymmetrical behavior on the electron and hole sides of the gate bias. A comparison with experimental data obtained by Kuzmenko et al demonstrates the good agreement.

Observation of an Electric-Field-Induced Band Gap in Bilayer Graphene by Infrared Spectroscopy

It has been predicted that application of a strong electric field perpendicular to the plane of bilayer graphene can induce a significant band gap. We have measured the optical conductivity of bilayer graphene with an efficient electrolyte top gate for a photon energy range of 0.2-0.7 eV. We see the emergence of new transitions as a band gap opens. A band gap approaching 200 meV is observed when an electric field approximately 1 V/nm is applied, inducing a carrier density of about 10(13) cm(-2)}. The magnitude of the band gap and the features observed in the infrared conductivity spectra are broadly compatible with calculations within a tight-binding model.

Direct observation of a widely tunable bandgap in bilayer graphene

The electronic bandgap is an intrinsic property of semiconductors and insulators that largely determines their transport and optical properties. As such, it has a central role in modern device physics and technology and governs the operation of semiconductor devices such as p-n junctions, transistors, photodiodes and lasers. A tunable bandgap would be highly desirable because it would allow great flexibility in design and optimization of such devices, in particular if it could be tuned by applying a variable external electric field. However, in conventional materials, the bandgap is fixed by their crystalline structure, preventing such bandgap control. Here we demonstrate the realization of a widely tunable electronic bandgap in electrically gated bilayer graphene. Using a dual-gate bilayer graphene field-effect transistor (FET) and infrared microspectroscopy, we demonstrate a gate-controlled, continuously tunable bandgap of up to 250 meV. Our technique avoids uncontrolled chemical doping and provides direct evidence of a widely tunable bandgap-spanning a spectral range from zero to mid-infrared-that has eluded previous attempts. Combined with the remarkable electrical transport properties of such systems, this electrostatic bandgap control suggests novel nanoelectronic and nanophotonic device applications based on graphene.

On the Possibility of Tunable-Gap Bilayer Graphene FET

We explore the device potential of a tunable-gap bilayer graphene (BG) FET exploiting the possibility of opening a bandgap in BG by applying a vertical electric field via independent gate operation. We evaluate device behavior using atomistic simulations based on the self-consistent solution of the Poisson and Schrodinger equations within the nonequilibrium Green's function formalism. We show that the concept works, but the bandgap opening is not strong enough to suppress band-to-band tunneling in order to obtain a sufficiently large I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> ratio for CMOS device operation.