Bandgap Engineering Of Graphene Research Articles

Electrical nature and surface enhanced Raman spectroscopy of Ag nanoparticles decorated graphene Sheet

The interaction between graphene and silver nanoparticles (AgNPs) is investigated by studying the surface-enhanced Raman spectroscopy (SERS) that shows a large enhancement of Raman signal from the AgNPs coated graphene compared to the bare graphene. In addition, the electrical nature of the sample is examined using a two-probe probe station to acquire current-voltage (I-V) characteristics exhibiting a semi-conducting current behaviour of the AgNPs-graphene heterojunctions, in contrast to a semi-metallic electronic behaviour observed for the pristine graphene. We propose band gap engineering of graphene from semi-metallic to semi-conducting via breaking of A-B sub-lattice symmetry due to deposition of AgNPs which is believed to be associated by causing deformations at the regions where graphene is in close proximity with the AgNPs. Furthermore, there is also a possibility of doping induced by AgNPs to graphene and thereby, emergence of a bandgap due to tuning of the electronic structure. In this work, we report these investigations which possess profound implications for application of graphene as a semi-conducting material.

Modulating the bandgap of Cr-intercalated bilayer graphene via combining the 18-electron rule and the 2D superatomic-molecule theory.

Bandgap engineering of graphene is of great significance for its potential applications in electronic devices. Herein, we used a sandwich compound Cr(C6H6)2 as the building block to construct Cr-intercalated bilayer graphene (BLG), namely a C12Cr monolayer. Chemical bonding analysis reveals that strong d-π interaction ensures π electrons of the graphene layers and d orbitals of the Cr atoms localized in C6CrC6 units to achieve the favored 18-electron rule, thus leading to a bandgap of 0.24 eV. Subsequently, a C48Cr monolayer with lower proportion of Cr is further designed using Cr(C54H18)2 as building units, where a newly developed two-dimensional (2D) superatomic-molecule theory is introduced to rationalize its electronic structure. The C48Cr monolayer not only satisfies the 18-electron rule, but also localizes extra π electrons to form two layers of 2D superatomic crystals composed of 2D superatoms (◊O and ◊N), resulting in a wider bandgap of 0.74 eV. This work opens an effective avenue to modulate the bandgap of BLG via combining the 18-electron rule and the 2D superatomic-molecule theory.

Electron transport in graphene nanoribbons with line defects

Bandgap engineering in graphene has been a hot topic in condensed matter physics. Although several line defects have been experimentally reported in graphene, the relationship between the bandgap engineering and the line defects has not yet been discussed. In this work, by combining the Green’s function method with the Landauer-Büttiker formula, we study theoretically the electron transport along disordered ZGNRs through taking into account three types of line defects which arise from random distribution of 4-8 rings. Our results show that although there exist electronic states around the Fermi energy of the disordered ZGNRs with randomly distributed line defects, all these electronic states are localized and a transmission gap appears around the Fermi energy. This localization phenomenon originates from the structural disorder induced by the randomly distributed line defects. To demonstrate the robustness of transmission gaps, we further calculate the conductance values of disordered ZGNR with different insertion probabilities and widths, finding that the size of transmission gap strongly depends upon the types of disorder, disorder degree, and width. When the disorder degree of line defects is low or the width of the nanoribbon is narrow, there is a notable difference in the size of the transmission gaps among the three types of disordered ZGNRs. As the width or disorder degree increases, the transmission gap size tends to be consistent. Like armchair ZGNRs, the transmission gap size decreases with the increase of width or disorder of ZGNR. Nonetheless, the openings of the transmission gaps in three types of disordered ZGNRs remain robust, regardless of variations in degree of disorder or width. These results are helpful in designing line-defect based nanodevices.

Nanohybrids with tunable band gap and low electron effective mass: Graphenes doped by multiple boron nitrogen domains

Two-dimensional (2-D) hexagonal boron–nitrogen–carbon (h-BNC) has long been considered as an important candidate material for the next generation of semiconductor electronic devices. Nevertheless, simply controlling the concentration of BN cannot effectively regulate the electronic properties. It is highly required to develop a more efficient doping strategy with the involvement of the geometrical morphology and arrangement of the BN domain. Inspired by recent experimental evidence that BN dopants preferentially form regularly shaped BN nanodomains on graphene by controlling precursors and high-temperature environments, in this work, for the first time, we investigate the effects of embedded BN domains of different shapes, sizes, and numbers on the electrical properties and thermodynamic stability of 2-D h-BNC nanohybrids using density functional theory.It is demonstrated that a doping mode with multiple small BN domains can effectively achieve a wide range of tunable band gap engineering of graphene, better than routinelysingle large BN domain.Meanwhile, multiple BN domain-modified graphene can maintain the excellent carrier mobility of intrinsic graphene.This unique behavior can be attributed to more boundary B/N atoms in the multiple BN domain doping h-BNC structures. Our simulation provides new conceptual insight into the doping configuration design for 2-D nanomaterials.

Bandgap engineering in massive-massless graphene superlattices

Bandgap engineering in graphene has become a fervent subject of research due to its relevance in technological applications. In this work, we proposed massive-massless graphene superlattices (MMGSLs) to tune the angle-dependent transmission properties. We have considered breaking symmetry substrates to generate massive and massless regions. We report an angular dependence of the transmission gaps and minibands as a function of the angle of incidence of the Dirac electrons in MMGSLs. Our results show that the transmission gaps and minibands have a trigonometric angular dependence based on the secant in the entire transmission energy range. In addition, the transmission gaps and minibands are well-defined at normal incidence, in contrast to gated graphene superlattices. These findings indicate that MMGSLs could be a possibility for bandgap engineering in graphene.

Band-gap engineering of graphene by Al doping and adsorption of Be and Br on impurity: A computational study

Graphene, being a gapless semiconductor, cannot be used in pristine form for optoelectronic applications such as solar cells. Therefore, it is necessary to tune a band-gap in pristine graphene. Density functional theory calculations have been performed to investigate the structural, electronic, and charge transfer mechanism of different impurities doped/adsorbed in graphene sheet. The results indicate that Al and Be doping can significantly modify the structural and electronic properties of graphene. Replacement of single C atom with Al atom induces a band-gap of 0.40 eV. Adopting different arrangements of Al, Be and Br employing doping and adsorption techniques we computed a variety of band-gap values, maximum value being 0.82 eV. Badar charge analysis indicates that the valence charge transmission takes place from less electronegative atoms towards atoms having higher electronegativity. The adsorption/doping of impurity atoms induce band-gap values in a significant wide range, which seems sufficient for its use in optoelectronic devices. Our results offer the opportunity to tune the electronic band structure of graphene for desired applications.

Realization of larger band gap opening of graphene and type-I band alignment with BN intercalation layer in graphene/ MX2 heterojunctions

Band gap engineering of graphene has been intensively studied, but it is still difficult to be realized effectively in practice, which limits its applications in optoelectronics. Here, by using first-principles methods, we predicate that the band gap of graphene can be obviously enlarged up to 0.83 eV and type-I band alignment is also realized in the $\text{graphene}/\mathrm{BN}/M{X}_{2}$ (M = Mo, W; X = S, Se) heterotrilayers when the interlayer distance is compressed. Moreover, the electrons and holes can be confined inside the gap opened graphene layer, and the staggered band alignment can be formed under the presence of electric field as well as the vertical pressure in the $\text{graphene}/\text{BN}/M{X}_{2}$ heterotrilayers. Our results may pay an effective route to tune the gap of graphene and broaden its applications in optoelectronic fields.

Ab-initio calculations of strain induced relaxed shape armchair graphene nanoribbon

An odd number of zigzag edges in armchair graphene nanoribbons and their mechanical properties (e.g., Young's modulus, Poisson ratio and shear modulus) have potential interest for bandgap engineering in graphene based optoelectronic devices. In this paper, we consider armchair graphene nanoribbons passivated with hydrogen at the armchair edges and then apply the strain for tuning the bandgaps. Using density functional theory calculations, our study finds that the precise control of strain can allow tuning the bandgap from semiconductor to mettalic and then again switching back to semiconductor. In addition, we also show that the strained graphene nanoribbon passivated with hydrogen molecules can have large out-of-plane deformations demonstrating the properties of relaxed shape graphene. We express the strain induced by hydrogen in terms of binding energy. Finally, we characterise the effect of strain on the mechanical properties that can be used for making straintronic devices based on graphene nanoribbons.

Revealing the Physicochemical Basis of Organic Solid-Solid Wetting Deposition: Casimir-like Forces, Hydrophobic Collapse, and the Role of the Zeta Potential.

Supramolecular self-assembly at the solid-solid interface enables the deposition and monolayer formation of insoluble organic semiconductors under ambient conditions. The underlying process, termed as the organic solid-solid wetting deposition (OSWD), generates two-dimensional adsorbates directly from dispersed three-dimensional organic crystals. This straightforward process has important implications in various fields of research and technology, such as in the domains of low-dimensional crystal engineering, the chemical doping and band gap engineering of graphene, and in the area of field-effect transistor fabrication. However, to date, lack of an in-depth understanding of the physicochemical basis of the OSWD prevented the identification of important parameters, essential to achieve a better control of the growth of monolayers and supramolecular assemblies with defined structures, sizes, and coverage areas. Here we propose a detailed model for the OSWD, derived from experimental and theoretical results that have been acquired by using the organic semiconductor quinacridone as an example system. The model reveals the vital role of the ζ potential and includes Casimir-like fluctuation-induced forces and the effect of dewetting in hydrophobic nanoconfinements. Based on our results, the OSWD of insoluble organic molecules can hence be applied to environmental friendly and low-cost dispersing agents, such as water. In addition, the model substantially enhances the ability to control the OSWD in terms of adsorbate structure and substrate coverage.

Competing Gap Opening Mechanisms of Monolayer Graphene and Graphene Nanoribbons on Strong Topological Insulators.

Graphene is a promising material for designing next-generation electronic and valleytronic devices, which often demand the opening of a bandgap in the otherwise gapless pristine graphene. To date, several conceptually different mechanisms have been extensively exploited to induce bandgaps in graphene, including spin-orbit coupling and inversion symmetry breaking for monolayer graphene, and quantum confinement for graphene nanoribbons (GNRs). Here, we present a multiscale study of the competing gap opening mechanisms in a graphene overlayer and GNRs proximity-coupled to topological insulators (TIs). We obtain sizable graphene bandgaps even without inversion symmetry breaking and identify the Kekulé lattice distortions caused by the TI substrates to be the dominant gap opening mechanism. Furthermore, Kekulé distorted armchair GNRs display intriguing nonmonotonous gap dependence on the nanoribbon width, resulting from the coexistence of quantum confinement, edge passivation, and Kekulé distortions. The present study offers viable new approaches for tunable bandgap engineering in graphene and GNRs.

Machine Learning Prediction of the Energy Gap of Graphene Nanoflakes Using Topological Autocorrelation Vectors.

The possibility of band gap engineering in graphene opens countless new opportunities for application in nanoelectronics. In this work, the energy gaps of 622 computationally optimized graphene nanoflakes were mapped to topological autocorrelation vectors using machine learning techniques. Machine learning modeling revealed that the most relevant correlations appear at topological distances in the range of 1 to 42 with prediction accuracy higher than 80%. The data-driven model can statistically discriminate between graphene nanoflakes with different energy gaps on the basis of their molecular topology.

Band gap engineering of graphene through quantum confinement and edge distortions

Based on the density functional theory approach we explore the chances endured by energy gap (EG) of semiconducting (armchair) graphene nanoribbons (AGNRs) when Stone-Wales (SW) defects are placed inside their lattices. Our results show that the AGNRs, which belong to the $$3\hbox {m} + 2$$ family experience an increase in their EG value. On the other hand, those belonging to 3m and $$3\hbox {m} + 1$$ families experience decrease in their EG. The maximum observed EG for pristine and distorted ribbons were $$\sim $$ 2.6 and $$\sim $$ 1.6 eV, respectively. Our results can be useful to understand the semiconducting properties of wider graphene nanoribbons which are already available experimentally.

Angle-dependent bandgap engineering in gated graphene superlattices

Graphene Superlattices (GSs) have attracted a lot of attention due to its peculiar properties as well as its possible technological implications. Among these characteristics we can mention: the extra Dirac points in the dispersion relation and the highly anisotropic propagation of the charge carriers. However, despite the intense research that is carried out in GSs, so far there is no report about the angular dependence of the Transmission Gap (TG) in GSs. Here, we report the dependence of TG as a function of the angle of the incident Dirac electrons in a rather simple Electrostatic GS (EGS). Our results show that the angular dependence of the TG is intricate, since for moderated angles the dependence is parabolic, while for large angles an exponential dependence is registered. We also find that the TG can be modulated from meV to eV, by changing the structural parameters of the GS. These characteristics open the possibility for an angle-dependent bandgap engineering in graphene.

Influence of defects on the electronic structures of bilayer graphene

Based on first-principles total-energy calculation, we investigate the electronic structures of bilayer graphene, one of which layers possesses atomic or topological defects, to explore the possibility of band gap engineering of graphene by means of physisorption of defective graphene. Our calculations show that the pristine graphene layer possesses a finite energy gap between bonding and antibonding π states because of the potential undulation caused by the other graphene layers with defects. We also found that the gap values strongly depend on the defect species and their mutual arrangement with respect to the pristine layer.

Quasi-Two-Dimensional SiC and SiC2: Interaction of Silicon and Carbon at Atomic Thin Lattice Plane

The band gap of graphene is nearly zero, and thus novel two-dimensional (2D) semiconductor and band gap engineering of graphene is highly desired for advanced optoelectronic applications. Herein, we have experimentally produced quasi-two-dimensional (quasi-2D) SiC by reaction between graphene and a silicon source, which was designed and supported by Born–Oppenheimer molecular dynamics simulations. The lateral length of the as-synthesized quasi-2D SiC is mainly in the range of 0.3–5 μm while the thickness is commonly below 10 nm. Quasi-2D SiC2 is also found as a byproduct, which is stable over 3 months in air atmosphere. The exciton binding energy of quasi-2D multilayers SiC can reach 0.23 eV while the band gap is around 3.72 eV. Additionally, in situ transmission electron microscopy has firmly proven that quasi-2D SiC can be synthesized through the reaction between graphene and silicon quantum dots. The first production of quasi-2D SiC and SiC2 makes the band gap engineering in the graphene lattice plane ...

Semiconducting Graphene on Silicon from First-Principles Calculations.

Graphene is a semimetal with zero band gap, which makes it impossible to turn electric conduction off below a certain limit. Transformation of graphene into a semiconductor has attracted wide attention. Owing to compatibility with Si technology, graphene adsorbed on a Si substrate is particularly attractive for future applications. However, to date there is little theoretical work on band gap engineering in graphene and its integration with Si technology. Employing first-principles calculations, we study the electronic properties of monolayer and bilayer graphene adsorbed on clean and hydrogen (H)-passivated Si (111)/Si (100) surfaces. Our calculation shows that the interaction between monolayer graphene and a H-passivated Si surface is weak, with the band gap remaining negligible. For bilayer graphene adsorbed onto a H-passivated Si surface, the band gap opens up to 108 meV owing to asymmetry introduction. In contrast, the interaction between graphene and a clean Si surface is strong, leading to formation of chemical bonds and a large band gap of 272 meV. Our results provide guidance for device designs based on integrating graphene with Si technology.

Band-gap engineering with a twist: Formation of intercalant superlattices in twisted graphene bilayers

Graphene-based materials have long been considered as promising building blocks for a new generation of high-frequency (terahertz) electronic devices, but their use is complicated by the lack of an intrinsic band gap in graphene itself. Here we exploit synthetically controllable incommensuration of twisted graphene bilayers as a scaffold for intercalation of alkali metal ions with the periodicity of the bilayer supercell. Systematic exploration of the energy profiles of the ions as a function of position suggests that the alkali metal ions aggregate commensurately with the symmetry of the twisted bilayer. The intercalated alkali metal ions act as a source of a periodic perturbation on the level of the bilayer supercell, which permits opening and engineering of a band gap between graphene's \ensuremath{\pi} bands. The twist angle between the graphene layers determines the structure and disorder of the intercalant sublattice and, consequently, the magnitude of the band gap. Appropriate choices of the intercalant and twist angle thus permit band-gap engineering in graphene. We offer arguments that the impact of intercalation on the all important charge mobility of graphene will be rather small.

Local strain effect on the band gap engineering of graphene by a first-principles study

We have systematically investigated the effect of local strain on electronic properties of graphene by first-principles calculations. Two major types of local strain, oriented along the zigzag and the armchair directions, have been studied. We find that local strain with a proper range and strength along the zigzag direction results in opening of significant band gaps in graphene, on the order of 10−1 eV; whereas, local strain along the armchair direction cannot open a significant band gap in graphene. Our results show that appropriate local strain can effectively open and tune the band gap in graphene; therefore, the electronic and transport properties of graphene can also be modified.

Band gap engineering of graphene with inter-layer embedded BN: From first principles calculations

Graphene is a promising material for the next generation electronics. However, its zero band gap limits its applications on electronic devices. Robust methods that modify graphene electrical properties by chemical doping are in great demand due to the potential applications of graphene on electronic devices. In this work, we explore BN doping behaviors with three different concentrations in graphene matrix by using first principles calculations. We calculate the band structure and find that the band gap opens up from zero in pristine graphene to 51, 78 and 93meV in 1%, 2% and 3% BN embedded graphene, respectively. The results of calculated band gap reveal that BN doped graphene exhibits an intrinsic semiconductor band structure characteristic, in which the Fermi level lies in the center of the forbidden gap. The normalized formation energy of BN as a function of BN concentration in graphene reveals that BN would prefer accumulating a big domain rather than a small domain in graphene.

Patterning, Characterization, and Chemical Sensing Applications of Graphene Nanoribbon Arrays Down to 5 nm Using Helium Ion Beam Lithography

Bandgap engineering of graphene is an essential step toward employing graphene in electronic and sensing applications. Recently, graphene nanoribbons (GNRs) were used to create a bandgap in graphene and function as a semiconducting switch. Although GNRs with widths of <10 nm have been achieved, problems like GNR alignment, width control, uniformity, high aspect ratios, and edge roughness must be resolved in order to introduce GNRs as a robust alternative technology. Here we report patterning, characterization, and superior chemical sensing of ultranarrow aligned GNR arrays down to 5 nm width using helium ion beam lithography (HIBL) for the first time. The patterned GNR arrays possess narrow and adjustable widths, high aspect ratios, and relatively high quality. Field-effect transistors were fabricated on such GNR arrays and temperature-dependent transport measurements show the thermally activated carrier transport in the GNR array structure. Furthermore, we have demonstrated exceptional NO2 gas sensitivity of the 5 nm GNR array devices down to parts per billion (ppb) levels. The results show the potential of HIBL fabricated GNRs for the electronic and sensing applications.