Showing papers on "Band gap published in 2012"
TL;DR: A low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight is reported.
Abstract: The energy costs associated with separating tightly bound excitons (photoinduced electron-hole pairs) and extracting free charges from highly disordered low-mobility networks represent fundamental losses for many low-cost photovoltaic technologies. We report a low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight. This "meso-superstructured solar cell" exhibits exceptionally few fundamental energy losses; it can generate open-circuit photovoltages of more than 1.1 volts, despite the relatively narrow absorber band gap of 1.55 electron volts. The functionality arises from the use of mesoporous alumina as an inert scaffold that structures the absorber and forces electrons to reside in and be transported through the perovskite.
9,158 citations
TL;DR: A bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness is reported, which has potential for high-frequency operation and large-scale integration.
Abstract: An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively Such devices have potential for high-frequency operation and large-scale integration
2,401 citations
TL;DR: In this paper, a quasi-particle composed of two electrons and a hole was identified in doped monolayer MoS2, which can be created with valley and spin polarized holes.
Abstract: Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties. In contrast to graphene, monolayer MoS2 is a non-centrosymmetric material with a direct energy gap. Strong photoluminescence, a current on-off ratio exceeding 10^8 in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here we report the spectroscopic identification in doped monolayer MoS2 of tightly bound negative trions, a quasi-particle composed of two electrons and a hole. These quasi-particles, which can be created with valley and spin polarized holes, have no analogue in other semiconducting materials. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up new avenues both for fundamental studies of many-body interactions and for opto-electronic and valleytronic applications in 2D atomic crystals.
1,719 citations
TL;DR: In this article, the authors describe the polycondensation of this structure, how to modify band positions and band gap by doping and copolymerization, and how to texture the organic solid to make it an effective photocatalyst.
Abstract: Polymeric graphitic carbon nitride (for simplicity, g-C3N4) is a layered material similar to graphene, being composed of only C, N, and some impurity H. Contrary to graphenes, g-C3N4 is a medium band gap semiconductor and an effective photocatalyst for a broad variety of reactions, and it possesses a high thermal and chemical stability In this Perspective, we describe the polycondensation of this structure, how to modify band positions and band gap by doping and copolymerization, and how to texture the organic solid to make it an effective photocatalyst. We then describe the photochemical splitting of water and some mild and selective photooxidation reactions catalyzed by g-C3N4.
1,449 citations
TL;DR: In this paper, the Bethe-Salpeter equation (BSE) was used to detect two strongly bound excitons below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band at the $K$ point of the Brillouin zone.
Abstract: Quasiparticle band structures and optical properties of MoS${}_{2}$, MoSe${}_{2}$, MoTe${}_{2}$, WS${}_{2}$, and WSe${}_{2}$ monolayers are studied using the GW approximation in conjunction with the Bethe-Salpeter equation (BSE). The inclusion of two-particle excitations in the BSE approach reveals the presence of two strongly bound excitons ($A$ and $B$) below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band at the $K$ point of the Brillouin zone. The transition energies for monolayer MoS${}_{2}$, in particular, are shown to be in excellent agreement with available absorption and photoluminescence measurements. Excitation energies for the remaining monolayers are predicted to lie in the range of 1--2 eV. Systematic trends are identified for quasiparticle band gaps, transition energies, and exciton binding energies within as well as across the Mo and W families of dichalcogenides. Overall, the results suggest that quantum confinement of carriers within monolayers can be exploited in conjunction with chemical composition to tune the optoelectronic properties of layered transition-metal dichalcogenides at the nanoscale.
1,282 citations
TL;DR: The fabrication of top-gate phototransistors based on a few-layered MoS(2) nanosheet with a transparent gate electrode exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection.
Abstract: We report on the fabrication of top-gate phototransistors based on a few-layered MoS2 nanosheet with a transparent gate electrode. Our devices with triple MoS2 layers exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection. The varied functionalities are attributed to energy gap modulation by the number of MoS2 layers. The photoelectric probing on working transistors with the nanosheets demonstrates that single-layer MoS2 has a significant energy bandgap of 1.8 eV, while those of double- and triple-layer MoS2 reduce to 1.65 and 1.35 eV, respectively.
1,247 citations
TL;DR: Ab initio quantum transport simulation of a dual-gated silicene field effect transistor confirms that the vertical electric field opens a transport gap, and a significant switching effect by an applied gate voltage is also observed, meaning biased single-layer silicenes and germanene can work effectively at room temperature as field effect transistors.
Abstract: By using ab initio calculations, we predict that a vertical electric field is able to open a band gap in semimetallic single-layer buckled silicene and germanene. The sizes of the band gap in both silicene and germanene increase linearly with the electric field strength. Ab initio quantum transport simulation of a dual-gated silicene field effect transistor confirms that the vertical electric field opens a transport gap, and a significant switching effect by an applied gate voltage is also observed. Therefore, biased single-layer silicene and germanene can work effectively at room temperature as field effect transistors.
1,229 citations
TL;DR: It is demonstrated that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion.
Abstract: Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe2, which shows stark contrast from the well-explored MoS2 where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe2 is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe2, identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices.
1,197 citations
TL;DR: In this paper, the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si atoms were analyzed.
Abstract: We report calculations of the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si atoms. The electric field produces a tunable band gap in the Dirac-type electronic spectrum, the gap being suppressed by a factor of about eight by the high polarizability of the system. At low electric fields, the interplay between this tunable band gap, which is specific to electrons on a honeycomb lattice, and the Kane-Mele spin-orbit coupling induces a transition from a topological to a band insulator, whereas at much higher electric fields silicene becomes a semimetal.
969 citations
TL;DR: The results suggest that mechanical strains reduce the band gap of semiconducting TMDs causing an direct-to-indirect band gap and a semiconductor- to-metal transition, and highlight the importance of tensile and pure shear strains in tuning the electronic properties of T MDs.
Abstract: Semiconducting transition metal dichalcogenides (TMDs) are emerging as the potential alternatives to graphene. As in the case of graphene, the monolayer of TMDs can easily be exfoliated using mechanical or chemical methods, and their properties can also be tuned. At the same time, semiconducting TMDs (MX2; M = Mo, W and X = S, Se, Te) possess an advantage over graphene in that they exhibit a band gap whose magnitude is appropriate for applications in the opto-electronic devices. Using ab initio simulations, we demonstrate that this band gap can be widely tuned by applying mechanical strains. While the electronic properties of graphene remain almost unaffected by tensile strains, we find TMDs to be sensitive to both tensile and shear strains. Moreover, compared to that of graphene, a much smaller amount of strain is required to vary the band gap of TMDs. Our results suggest that mechanical strains reduce the band gap of semiconducting TMDs causing an direct-to-indirect band gap and a semiconductor-to-metal...
796 citations
TL;DR: The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies.
Abstract: Graphene, a single layer of carbon atoms in a honeycomb lattice, offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. Its unique form factor and exceptional physical properties have the potential to enable an entirely new generation of technologies beyond the limits of conventional materials. The extraordinarily high carrier mobility and saturation velocity can enable a fast switching speed for radio-frequency analog circuits. Unadulterated graphene is a semi-metal, incapable of a true off-state, which typically precludes its applications in digital logic electronics without bandgap engineering. The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already shown the numerous advantages and novel functionality that graphene provides.
TL;DR: Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a TCI, a new class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection.
Abstract: Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection. In this study we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.
TL;DR: In this article, the authors outline the current understanding of two general aspects of optical response of graphene: optical absorption and light emission, and show that optical absorption in graphene is dominated by intraband transitions at low photon energies and by interband transitions at higher energies (from mid-infrared to ultraviolet).
TL;DR: The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage as mentioned in this paper, and various alloys have been employed in the practical implementation of these devices.
Abstract: The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage. Nanostructured materials and certain alloys have been employed in the practical implementation of these devices. This Progress Article reviews the range of different approaches and discusses how to resolve the remaining technical issues.
01 Jan 2012
TL;DR: In this article, the authors demonstrate the importance of tensile and pure shear strains in tuning the electronic properties of semiconducting transition metaldichalcogenides (TMDs) by illustrating a substantial impact of the strain on semiconductor-to-metal transition.
Abstract: Semiconducting transitionmetaldichalcogenides (TMDs) are emerging as the potential alternatives to gra- phene. As in the case of gra- phene, the monolayer of TMDs can easily be exfoliated using me- chanical or chemical methods, and their properties can also be tuned. At the same time, semiconducting TMDs(MX2;M=Mo,WandX=S, Se, Te) possess an advantage over grapheneinthattheyexhibitabandgapwhosemagnitudeisappropriateforapplicationsintheopto- electronicdevices.Usingabinitiosimulations,wedemonstratethatthisbandgapcanbewidelytuned by applyingmechanicalstrains.While theelectronic propertiesof grapheneremain almost unaffected by tensile strains, we find TMDs to be sensitive to both tensile and shear strains. Moreover, compared to that of graphene, a much smaller amount of strain is required to vary the band gap of TMDs. Our results suggest that mechanical strains reduce the band gap of semiconducting TMDs causing an direct-to-indirect band gap and a semiconductor-t o-metal transition. These transitions, however, significantly depend on the type of applied strain and the type of chalcogenide atoms. The diffuse nature of heavier chalcogenides require relatively more tensile and less shear strain (when the monolayerisexpandediny-directionandcompressedinx-direction) toattainadirect-to-indirectband gap transition. In addition, our results demonstrate that the homogeneous biaxial tensile strain of around 10% leads to semiconductor-to-metal transition in all semiconducting TMDs, while through pureshearstrainthistransitioncanonlybeachievedbyexpandingandcompressingthemonolayerof MTe2in the y-andx-directions, respectively. Our results highlight the importance of tensile and pure shear strains in tuning the electronic properties of TMDs by illustrating a substantial impact of the strain on going from MS2 to MSe2 to MTe2.
TL;DR: In this paper, the use of vacuum co-evaporation to produce Cu2ZnSnSe4 photovoltaic devices with 9.15% total area efficiency is described.
TL;DR: This work generates single layers in arbitrary shapes and patterns with feature sizes down to 200 nm and shows that the resulting two-dimensional crystals have optical and electronic properties comparable to that of pristine exfoliated MoS(2) single layers.
Abstract: Single-layer MoS2 is an attractive semiconducting analogue of graphene that combines high mechanical flexibility with a large direct bandgap of 1.8 eV. On the other hand, bulk MoS2 is an indirect bandgap semiconductor similar to silicon, with a gap of 1.2 eV, and therefore deterministic preparation of single MoS2 layers is a crucial step toward exploiting the large direct bandgap of monolayer MoS2 in electronic, optoelectronic, and photovoltaic applications. Although mechanical and chemical exfoliation methods can be used to obtain high quality MoS2 single layers, the lack of control in the thickness, shape, size, and position of the flakes limits their usefulness. Here we present a technique for controllably thinning multilayered MoS2 down to a single-layer two-dimensional crystal using a laser. We generate single layers in arbitrary shapes and patterns with feature sizes down to 200 nm and show that the resulting two-dimensional crystals have optical and electronic properties comparable to that of prist...
TL;DR: In this article, an effective structural doping approach has been described to modify the photoelectrochemical properties of g-C3N4 by doping with nonmetal (sulfur or phosphorus) impurities.
Abstract: An effective structural doping approach has been described to modify the photoelectrochemical properties of g-C3N4 by doping with nonmetal (sulfur or phosphorus) impurities. Here, the substitutional and interstitial doped models of g-C3N4 systems were constructed with different doped sites, and then their dopant formation energies and electronic properties were performed to study the stability and visible-light photoactivity using first-principles density functional theory, respectively. Our results have identified that an S atom preferentially substitutes for the edge N atom of g-C3N4; however, a P atom preferentially situates the interstitial sites of in-planar of g-C3N4. Furthermore, it is demonstrated that the doping with nonmetal impurities reduces the energy gap to enhance the visible-light absorption of g-C3N4. The increased dispersion of the contour distribution of the HOMO and LUMO brought by doping facilitates the enhancement of the carrier mobility, while the noncoplanar HOMO and LUMO favor the...
TL;DR: In this article, the performance of three distinctly different phases, Cu2O, Cu4O3, and CuO, of this binary semiconductor can be prepared by thin-film deposition techniques, which differ in the oxidation state of copper.
Abstract: Copper-oxide compound semiconductors provide a unique possibility to tune the optical and electronic properties from insulating to metallic conduction, from bandgap energies of 2.1 eV to the infrared at 1.40 eV, i.e., right into the middle of the efficiency maximum for solar-cell applications. Three distinctly different phases, Cu2O, Cu4O3, and CuO, of this binary semiconductor can be prepared by thin-film deposition techniques, which differ in the oxidation state of copper. Their material properties as far as they are known by experiment or predicted by theory are reviewed. They are supplemented by new experimental results from thin-film growth and characterization, both will be critically discussed and summarized. With respect to devices the focus is on solar-cell performances based on Cu2O. It is demonstrated by photoelectron spectroscopy (XPS) that the heterojunction system p-Cu2O/n-AlGaN is much more promising for the application as efficient solar cells than that of p-Cu2O/n-ZnO heterojunction devices that have been favored up to now.
TL;DR: In this paper, the structural and electronic data relevant for the solar cells were summarised and the authors concluded that the equilibrium structure of both Cu2ZnSnS4 and Cu2znSnSe4 is the kesterite structure.
Abstract: Kesterite materials (Cu2ZnSn(S,Se)4) are made from non-toxic, earth-abundant and low-cost raw materials. We summarise here the structural and electronic material data relevant for the solar cells. The equilibrium structure of both Cu2ZnSnS4 and Cu2ZnSnSe4 is the kesterite structure. However, the stannite structure has only a slightly lower binding energy. Because the band gap of the stannite is predicted to be about 100 meV lower than the kesterite band gap, any admixture of stannite will hurt the solar cells. The band gaps of Cu2ZnSnS4 and Cu2ZnSnSe4 are 1.5 and 1.0 eV, respectively. Hardly any experiments on defects are available. Theoretically, the CuZn antisite acceptor is predicted as the most probable defect. The existence region of the kesterite phase is smaller compared with that of chalcopyrites. This makes secondary phases a serious challenge in the development of solar cells. Copyright © 2012 John Wiley & Sons, Ltd.
TL;DR: An inhomogeneous planar substrate (g-C(3)N(4)) promotes electron-rich and hole-rich regions, i.e., forming a well-defined electron-hole puddle, on the supported graphene layer, which can potentially allow overcoming the graphene's band gap hurdle in constructing field effect transistors.
Abstract: Opening up a band gap and finding a suitable substrate material are two big challenges for building graphene-based nanodevices. Using state-of-the-art hybrid density functional theory incorporating long-range dispersion corrections, we investigate the interface between optically active graphitic carbon nitride (g-C(3)N(4)) and electronically active graphene. We find an inhomogeneous planar substrate (g-C(3)N(4)) promotes electron-rich and hole-rich regions, i.e., forming a well-defined electron-hole puddle, on the supported graphene layer. The composite displays significant charge transfer from graphene to the g-C(3)N(4) substrate, which alters the electronic properties of both components. In particular, the strong electronic coupling at the graphene/g-C(3)N(4) interface opens a 70 meV gap in g-C(3)N(4)-supported graphene, a feature that can potentially allow overcoming the graphene's band gap hurdle in constructing field effect transistors. Additionally, the 2-D planar structure of g-C(3)N(4) is free of dangling bonds, providing an ideal substrate for graphene to sit on. Furthermore, when compared to a pure g-C(3)N(4) monolayer, the hybrid graphene/g-C(3)N(4) complex displays an enhanced optical absorption in the visible region, a promising feature for novel photovoltaic and photocatalytic applications.
TL;DR: Tran et al. as mentioned in this paper examined the possibility to further improve over the original TB-mBJ potential by either reparametrizing its coefficients using a larger test set of solids or defining a parametrization for small/medium-size band-gap semiconductors only.
Abstract: The modified Becke-Johnson exchange potential [F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)] (TB-mBJ) yields very accurate electronic band structures and gaps for various types of semiconductors and insulators (e.g., $sp$ semiconductors, noble-gas solids, and transition-metal oxides). However, the TB-mBJ potential has, for a few groups of solids, the tendency to underestimate the band gap. This has led us to examine the possibility to further improve over the original TB-mBJ potential by either reparametrizing its coefficients using a larger test set of solids or defining a parametrization for small-/medium-size band-gap semiconductors only. We also checked alternatives to the average of $|\ensuremath{
abla}\ensuremath{\rho}|/\ensuremath{\rho}$ in the unit cell for the determination of parameter $c$, which determines the amount of the screening contribution. Among these different possibilities, the best one seems to be a reparametrization of the coefficients, which leads to a much more balanced description of the band gaps.
TL;DR: In this paper, the electronic structure of a single MoS2 monolayer is investigated with all electron first-principles calculations based on Kohn Sham Density Functional Theory and variational treatment of spin-orbital coupling.
TL;DR: In this paper, the effects of quantum confinement on the electronic structure of monolayer transition metal dichalcogenides have been investigated using the Bethe-Salpeter equation.
Abstract: Using $GW$ first-principles calculations for few-layer and bulk MoS${}_{2}$, we study the effects of quantum confinement on the electronic structure of this layered material. By solving the Bethe-Salpeter equation, we also evaluate the exciton energy in these systems. Our results are in excellent agreement with the available experimental data. Exciton binding energy is found to dramatically increase from 0.1 eV in the bulk to 1.1 eV in the monolayer. The fundamental band gap increases as well, so that the optical transition energies remain nearly constant. We also demonstrate that environments with different dielectric constants have a profound effect on the electronic structure of the monolayer. Our results can be used for engineering the electronic properties of MoS${}_{2}$ and other transition-metal dichalcogenides and may explain the experimentally observed variations in the mobility of monolayer MoS${}_{2}$.
TL;DR: The realization of ambipolar field-effect transistors by coupling exfoliated thin flakes of tungsten disulfide with an ionic liquid dielectric shows ideal electrical characteristics, including very steep subthreshold slopes for both electrons and holes and extremely low OFF-state currents.
Abstract: We realized ambipolar field-effect transistors by coupling exfoliated thin flakes of tungsten disulfide (WS2) with an ionic liquid dielectric. The devices show ideal electrical characteristics, including very steep subthreshold slopes for both electrons and holes and extremely low OFF-state currents. Thanks to these ideal characteristics, we determine with high precision the size of the band gap of WS2 directly from the gate-voltage dependence of the source-drain current. Our results demonstrate how a careful use of ionic liquid dielectrics offers a powerful strategy to study quantitatively the electronic properties of nanoscale materials.
TL;DR: In this paper, it was shown that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a topological crystalline insulator for x = 0.23.
Abstract: Topological insulators are a novel class of quantum materials in which time-reversal symmetry, relativistic (spin-orbit) effects and an inverted band structure result in electronic metallic states on the surfaces of bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical proposals have suggested the existence of topological crystalline insulators, a novel class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in topological protection [1,2]. In this study, we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a topological crystalline insulator for x=0.23. Temperature-dependent magnetotransport measurements and angle-resolved photoelectron spectroscopy demonstrate that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a topological crystalline insulator. These experimental findings add a new class to the family of topological insulators. We expect these results to be the beginning of both a considerable body of additional research on topological crystalline insulators as well as detailed studies of topological phase transitions.
TL;DR: In this review, a summary will be given on a series of low band gap polycyclic hydrocarbons about their synthesis, physical properties and material applications.
Abstract: Low band gap (Eg < 1.5 eV) polycyclic hydrocarbons have become one of the most important types of materials for many applications, for example, as semiconductors in organic field effect transistors (OFETs), as light-harvesting dyes in organic solar cells and photodetectors, as near infrared (NIR) fluorescent probes in high resolution bio-imaging and bio-sensing, and as chromophores in non-linear optics. The benzenoid polycyclic hydrocarbons as nano-sized graphene fragments also serve as perfect model compounds to understand the fundamental structure–property relationship of graphene. The ground state of these molecules can be described as either a closed-shell or an open-shell structure on the basis of their molecular size and edge structure. In this review, a summary will be given on a series of low band gap polycyclic hydrocarbons about their synthesis, physical properties and material applications.
TL;DR: In this paper, first principles calculations of the electronic structure of monolayer 1H-MX2 (M = Mo, W; X = S, Se, Te), using the pseudopotential and numerical atomic orbital basis sets based methods within the local density approximation.
Abstract: We report first principles calculations of the electronic structure of monolayer 1H-MX2 (M = Mo, W; X = S, Se, Te), using the pseudopotential and numerical atomic orbital basis sets based methods within the local density approximation. Electronic band structure and density of states calculations found that the states around the Fermi energy are mainly due to metal d states. From partial density of states we find a strong hybridisation between metal d and chalcogen p states below the Fermi energy. All studied compounds in this work have emerged as new direct band gap semiconductors. The electronic band gap is found to decrease as one goes from sulphides to the tellurides of both Mo and W. Reducing the slab thickness systematically from bulk to monolayers causes a blue shift in the band gap energies, resulting in tunability of the electronic band gap. The magnitudes of the blue shift in the band gap energies are found to be 1.14 eV, 1.16 eV, 0.78 eV, 0.64, 0.57 eV and 0.37 eV for MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, respectively, as we go from bulk phase (indirect band gap) to monolayer limit (direct band gap). This tunability in the electronic band gap and transitions from indirect to direct band make these materials potential candidates for the fabrication of optoelectronic devices.
TL;DR: The electronic band gap and dispersion of the occupied electronic bands of atomically precise graphene nanoribbons fabricated via on-surface synthesis are reported on and are in quantitative agreement with theoretical predictions that include image charge corrections accounting for screening by the metal substrate and confirm the importance of electron-electron interactions in graphene nan oribbons.
Abstract: Some of the most intriguing properties of graphene are predicted for specifically designed nanostructures such as nanoribbons. Functionalities far beyond those known from extended graphene systems include electronic band gap variations related to quantum confinement and edge effects, as well as localized spin-polarized edge states for specific edge geometries. The inability to produce graphene nanostructures with the needed precision, however, has so far hampered the verification of the predicted electronic properties. Here, we report on the electronic band gap and dispersion of the occupied electronic bands of atomically precise graphene nanoribbons fabricated via on-surface synthesis. Angle-resolved photoelectron spectroscopy and scanning tunneling spectroscopy data from armchair graphene nanoribbons of width N = 7 supported on Au(111) reveal a band gap of 2.3 eV, an effective mass of 0.21 m0 at the top of the valence band, and an energy-dependent charge carrier velocity reaching 8.2 × 105 m/s in the li...
TL;DR: In this article, the authors present a technique for controllably thinning multilayered MoS2 down to a single-layer two-dimensional crystal using a laser, which has optical and electronic properties comparable to pristine exfoliated single layers.
Abstract: Single-layer MoS2 is an attractive semiconducting analogue of graphene that combines high mechanical flexibility with a large direct bandgap of 1.8 eV. On the other hand, bulk MoS2 is an indirect bandgap semiconductor similar to silicon, with a gap of 1.2 eV, and therefore deterministic preparation of single MoS2 layers is a crucial step towards exploiting the large direct bandgap of monolayer MoS2 in electronic, optoelectronic, and photovoltaic applications. Although mechanical and chemical exfoliation methods can be used to obtain high quality MoS2 single-layers, the lack of control in the thickness, shape, size, and position of the flakes limits their usefulness. Here we present a technique for controllably thinning multilayered MoS2 down to a single-layer two-dimensional crystal using a laser. We generate single layers in arbitrary shapes and patterns with feature sizes down to 200 nm, and show that the resulting two-dimensional crystals have optical and electronic properties comparable to that of pristine exfoliated MoS2 single layers.