Showing papers on "Band gap published in 2011"
TL;DR: Above an annealing temperature of 300 °C, chemically exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mechanically exfoliate monolayers, indicating that their semiconducting properties are largely restored.
Abstract: A two-dimensional crystal of molybdenum disulfide (MoS2) monolayer is a photoluminescent direct gap semiconductor in striking contrast to its bulk counterpart. Exfoliation of bulk MoS2 via Li intercalation is an attractive route to large-scale synthesis of monolayer crystals. However, this method results in loss of pristine semiconducting properties of MoS2 due to structural changes that occur during Li intercalation. Here, we report structural and electronic properties of chemically exfoliated MoS2. The metastable metallic phase that emerges from Li intercalation was found to dominate the properties of as-exfoliated material, but mild annealing leads to gradual restoration of the semiconducting phase. Above an annealing temperature of 300 °C, chemically exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mechanically exfoliated monolayers, indicating that their semiconducting properties are largely restored.
3,403 citations
TL;DR: It is demonstrated that silicene with topologically nontrivial electronic structures can realize the quantum spin Hall effect (QSHE) by exploiting adiabatic continuity and the direct calculation of the Z(2) topological invariant.
Abstract: We investigate the spin-orbit opened energy gap and the band topology in recently synthesized silicene as well as two-dimensional low-buckled honeycomb structures of germanium using first-principles calculations. We demonstrate that silicene with topologically nontrivial electronic structures can realize the quantum spin Hall effect (QSHE) by exploiting adiabatic continuity and the direct calculation of the Z(2) topological invariant. We predict that the QSHE can be observed in an experimentally accessible low temperature regime in silicene with the spin-orbit band gap of 1.55 meV, much higher than that of graphene. Furthermore, we find that the gap will increase to 2.9 meV under certain pressure strain. Finally, we also study germanium with a similar low-buckled stable structure, and predict that spin-orbit coupling opens a band gap of 23.9 meV, much higher than the liquid nitrogen temperature.
1,940 citations
TL;DR: This review encompasses several advancements made in these aspects of titania, and also some of the new physical insights related to the charge transfer events like charge carrier generation, trapping, detrapping, and their transfer to surface are discussed for each strategy of the modified titania to support the conclusions derived.
Abstract: Titania is one of the most widely used benchmark standard photocatalysts in the field of environmental applications. However, the large band gap of titania and massive recombination of photogenerated charge carriers limit its overall photocatalytic efficiency. The former can be overcome by modifying the electronic band structure of titania including various strategies like coupling with a narrow band gap semiconductor, metal ion/nonmetal ion doping, codoping with two or more foreign ions, surface sensitization by organic dyes or metal complexes, and noble metal deposition. The latter can be corrected by changing the surface properties of titania by fluorination or sulfation or by the addition of suitable electron acceptors besides molecular oxygen in the reaction medium. This review encompasses several advancements made in these aspects, and also some of the new physical insights related to the charge transfer events like charge carrier generation, trapping, detrapping, and their transfer to surface are d...
1,728 citations
IBM1
TL;DR: Heusler compounds as discussed by the authors are a remarkable class of intermetallic materials with 1:1:1 or 2:1-1 composition comprising more than 1500 members, and their properties can easily be predicted by the valence electron count.
1,675 citations
TL;DR: A class of three-dimensional "topological crystalline insulators" which have metallic surface states with quadratic band degeneracy on high symmetry crystal surfaces is found.
Abstract: The recent discovery of topological insulators has revived interest in the band topology of insulators. In this Letter, we extend the topological classification of band structures to include certain crystal point group symmetry. We find a class of three-dimensional ``topological crystalline insulators'' which have metallic surface states with quadratic band degeneracy on high symmetry crystal surfaces. These topological crystalline insulators are the counterpart of topological insulators in materials without spin-orbit coupling. Their band structures are characterized by new topological invariants. We hope this work will enlarge the family of topological phases in band insulators and stimulate the search for them in real materials.
1,641 citations
TL;DR: In this article, it was shown that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure at the nanoscale, and the properties of related TmS2 nanolayers (Tm = W, Nb, Re) were studied.
Abstract: Bulk MoS2, a prototypical layered transition-metal dichalcogenide, is an indirect band gap semiconductor. Reducing its size to a monolayer, MoS2 undergoes a transition to the direct band semiconductor. We support this experimental observation by first principles calculations and show that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure at the nanoscale. We further studied the properties of related TmS2 nanolayers (Tm = W, Nb, Re) and show that the isotopological WS2 exhibits similar electronic properties, while NbS2 and ReS2 remain metallic independent on size.
1,532 citations
TL;DR: The resulting fluorinated polymer PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells.
Abstract: Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (∼2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT−FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C61-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT−FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT−FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, bes...
1,463 citations
TL;DR: In this paper, a summary of chemical doping of graphene aimed at tuning the electronic properties of graphene is presented, which will be beneficial to designing high performance electronic devices based on chemically doped graphene.
Abstract: Recently, a lot of effort has been focused on improving the performance and exploring the electric properties of graphene. This article presents a summary of chemical doping of graphene aimed at tuning the electronic properties of graphene. p-Type and n-type doping of graphene achieved through surface transfer doping or substitutional doping and their applications based on doping are reviewed. Chemical doping for band gap tuning in graphene is also presented. It will be beneficial to designing high performance electronic devices based on chemically doped graphene.
1,447 citations
TL;DR: This simulation results show that while MoS(2) transistors may not be ideal for high-performance applications due to heavier electron effective mass and a lower mobility, they can be an attractive alternative for low power applications thanks to the large band gap and the excellent electrostatic integrity inherent in a two-dimensional system.
Abstract: Monolayer molybdenum disulfide (MoS2), unlike its bulk form, is a direct band gap semiconductor with a band gap of 1.8 eV. Recently, field-effect transistors have been demonstrated experimentally using a mechanically exfoliated MoS2 monolayer, showing promising potential for next generation electronics. Here we project the ultimate performance limit of MoS2 transistors by using nonequilibrium Green’s function based quantum transport simulations. Our simulation results show that the strength of MoS2 transistors lies in large ON–OFF current ratio (>1010), immunity to short channel effects (drain-induced barrier lowering ∼10 mV/V), and abrupt switching (subthreshold swing as low as 60 mV/decade). Our comparison of monolayer MoS2 transistors to the state-of-the-art III–V materials based transistors, reveals that while MoS2 transistors may not be ideal for high-performance applications due to heavier electron effective mass (m* = 0.45m0) and a lower mobility, they can be an attractive alternative for low power...
1,397 citations
TL;DR: Scanning tunnelling microscopy is used to show that graphene conforms to hBN, as evidenced by the presence of Moiré patterns, but contrary to predictions, this conformation does not lead to a sizeable band gap because of the misalignment of the lattices.
Abstract: Using boron nitride as a substrate for graphene has been suggested as a promising way to reduce the disorder in graphene caused by space fluctuations. It is now shown by scanning tunnelling microscopy that graphene conforms perfectly to boron nitride and the charge fluctuations are minimal compared with the conventionally used substrate, silica. Boron nitride could really be the natural graphene substrate. Graphene has demonstrated great promise for future electronics technology as well as fundamental physics applications because of its linear energy–momentum dispersion relations which cross at the Dirac point1,2. However, accessing the physics of the low-density region at the Dirac point has been difficult because of disorder that leaves the graphene with local microscopic electron and hole puddles3,4,5. Efforts have been made to reduce the disorder by suspending graphene, leading to fabrication challenges and delicate devices which make local spectroscopic measurements difficult6,7. Recently, it has been shown that placing graphene on hexagonal boron nitride (hBN) yields improved device performance8. Here we use scanning tunnelling microscopy to show that graphene conforms to hBN, as evidenced by the presence of Moire patterns. However, contrary to predictions9,10, this conformation does not lead to a sizeable band gap because of the misalignment of the lattices. Moreover, local spectroscopy measurements demonstrate that the electron–hole charge fluctuations are reduced by two orders of magnitude as compared with those on silicon oxide. This leads to charge fluctuations that are as small as in suspended graphene6, opening up Dirac point physics to more diverse experiments.
1,221 citations
TL;DR: In this paper, a heterojunction electrode was fabricated by layer-by-layer deposition of WO3 and BiVO4 on a conducting glass, and investigated for photoelectrochemical water oxidation under simulated solar light.
Abstract: Heterojunction electrodes were fabricated by layer-by-layer deposition of WO3 and BiVO4 on a conducting glass, and investigated for photoelectrochemical water oxidation under simulated solar light. The electrode with the optimal composition of four layers of WO3 covered by a single layer of BiVO4 showed enhanced photoactivity by 74% relative to bare WO3 and 730% relative to bare BiVO4. According to the flat band potential and optical band gap measurements, both semiconductors can absorb visible light and have band edge positions that allow the transfer of photoelectrons from BiVO4 to WO3. The electrochemical impedance spectroscopy revealed poor charge transfer characteristics of BiVO4, which accounts for the low photoactivity of bare BiVO4. The measurements of the incident photon-to-current conversion efficiency spectra showed that the heterojunction electrode utilized effectively light up to 540 nm covering absorption by both WO3 and BiVO4 layers. Thus, in heterojunction electrodes, the photogenerated electrons in BiVO4 are transferred to WO3 layers with good charge transport characteristics and contribute to the high photoactivity. They combine merits of the two semiconductors, i.e. excellent charge transport characteristics of WO3 and good light absorption capability of BiVO4 for enhanced photoactivity.
TL;DR: Using density functional theory coupled with Boltzmann transport equation with relaxation time approximation, the electronic structure is investigated and the charge mobility for a new carbon allotrope, the graphdiyne for both the sheet and nanoribbons is predicted.
Abstract: Using density functional theory coupled with Boltzmann transport equation with relaxation time approximation, we investigate the electronic structure and predict the charge mobility for a new carbon allotrope, the graphdiyne for both the sheet and nanoribbons. It is shown that the graphdiyne sheet is a semiconductor with a band gap of 0.46 eV. The calculated in-plane intrinsic electron mobility can reach the order of 10(5) cm(2)/(V s) at room temperature, while the hole mobility is about an order of magnitude lower.
TL;DR: This work creates an sp2-hybridized one-atom-thick flat carbon membrane with a random arrangement of polygons, including four-membered carbon rings that possess a band gap, which may open new possibilities for engineering graphene-based electronic devices.
Abstract: While crystalline two-dimensional materials have become an experimental reality during the past few years, an amorphous 2D material has not been reported before. Here, using electron irradiation we create an $s{p}^{2}$-hybridized one-atom-thick flat carbon membrane with a random arrangement of polygons, including four-membered carbon rings. We show how the transformation occurs step by step by nucleation and growth of low-energy multivacancy structures constructed of rotated hexagons and other polygons. Our observations, along with first-principles calculations, provide new insights to the bonding behavior of carbon and dynamics of defects in graphene. The created domains possess a band gap, which may open new possibilities for engineering graphene-based electronic devices.
TL;DR: It has been established that a postannealing of N-graphene after gold intercalation causes a conversion of the N environment from pyridinic to graphitic, allowing to obtain more than 80% of all embedded nitrogen in graphitic form, which is essential for the electron doping in graphene.
Abstract: A novel strategy for efficient growth of nitrogen-doped graphene (N-graphene) on a large scale from s-triazine molecules is presented. The growth process has been unveiled in situ using time-dependent photoemission. It has been established that a postannealing of N-graphene after gold intercalation causes a conversion of the N environment from pyridinic to graphitic, allowing to obtain more than 80% of all embedded nitrogen in graphitic form, which is essential for the electron doping in graphene. A band gap, a doping level of 300 meV, and a charge-carrier concentration of ∼8 × 1012 electrons per cm2, induced by 0.4 atom % of graphitic nitrogen, have been detected by angle-resolved photoemission spectroscopy, which offers great promise for implementation of this system in next generation electronic devices.
TL;DR: A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing in semiconductor nanocrystals, enabling control of the band gap and Fermi energy.
Abstract: Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices.
TL;DR: In this article, the authors investigate band-gap tuning in bilayer transition-metal dichalcogenides by external electric fields applied perpendicular to the layers, and show that the fundamental band gap of MoS, MoSe, MoTe, and WS bilayer structures continuously decreases with increasing applied electric fields, eventually rendering them metallic.
Abstract: We investigate band-gap tuning in bilayer transition-metal dichalcogenides by external electric fields applied perpendicular to the layers. Using density functional theory, we show that the fundamental band gap of MoS${}_{2}$, MoSe${}_{2}$, MoTe${}_{2}$, and WS${}_{2}$ bilayer structures continuously decreases with increasing applied electric fields, eventually rendering them metallic. We interpret our results in the light of the giant Stark effect and obtain a robust relationship, which is essentially characterized by the interlayer spacing, for the rate of change of band gap with applied external field. Our study expands the known space of layered materials with widely tunable band gaps beyond the classic example of bilayer graphene and suggests potential directions for fabrication of novel electronic and photonic devices.
TL;DR: In this article, the effect of interlayer interactions on the band structure and density of states using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof was investigated.
Abstract: Molybdenite (MoS2) undergoes a transition from an indirect to direct gap semiconductor exhibiting strong photoluminescence when confined in a 2D monolayer. We investigate the effect of interlayer interactions on the band structure and density of states using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof. We show that for the bulk and monolayer systems, our short-range screened hybrid functional produces band gaps in good agreement with experiment. Our functional includes only interlayer interactions of non-van der Waals origin, predicts properties consistent with recent experiments, and provides predictions for few-layered systems.
TL;DR: Two unexpected orthorhombic high-pressure structures Aba2-40 and Cmca-56 are reported, by using a newly developed particle swarm optimization technique on crystal structure prediction, and it is predicted that a local trigonal planar structural motif adopted by CmCA-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.
Abstract: Under high pressure, "simple" lithium (Li) exhibits complex structural behavior, and even experiences an unusual metal-to-semiconductor transition, leading to topics of interest in the structural polymorphs of dense Li. We here report two unexpected orthorhombic high-pressure structures Aba2-40 (40 atoms/cell, stable at 60-80 GPa) and Cmca-56 (56 atoms/cell, stable at 185-269 GPa), by using a newly developed particle swarm optimization technique on crystal structure prediction. The Aba2-40 having complex 4- and 8-atom layers stacked along the b axis is a semiconductor with a pronounced band gap >0.8 eV at 70 GPa originating from the core expulsion and localization of valence electrons in the voids of a crystal. We predict that a local trigonal planar structural motif adopted by Cmca-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.
TL;DR: In this article, the functionalization of the two-dimensional single-layer MoS2 structure through adatom adsorption and vacancy defect creation was studied based on first-principles plane-wave calculations.
Abstract: Based on first-principles plane-wave calculations, we studied the functionalization of the two-dimensional single-layer MoS2 structure through adatom adsorption and vacancy defect creation. Minimum-energy adsorption sites were determined for 16 different adatoms, each of which gives rise to diverse properties. Bare, single-layer MoS2, which is normally a nonmagnetic, direct-band-gap semiconductor, attains a net magnetic moment upon adsorption of specific transition-metal atoms, as well as silicon and germanium atoms. The localized donor and acceptor states in the band gap expand the utilization of MoS2 in nanoelectronics and spintronics. Specific adatoms, such as C and O, attain significant excess charge upon adsorption onto single-layer MoS2, which might be useful for tribological applications. Each MoS2 triple vacancy created in a single layer of MoS2 gives rise to a net magnetic moment, whereas other vacancy defects related to Mo and S atoms do not influence the nonmagnetic ground state. The present re...
TL;DR: In this article, infrared transmission measurements indicate both situations are possible depending on the stacking of the layers of monolayer and trilayer graphene, and both of them can be controlled by an electric field.
Abstract: Monolayer graphene has no electronic band gap. Bilayer graphene does, and can be controlled by an electric field. And for trilayer graphene, infrared transmission measurements indicate both situations are possible depending on the stacking of the layers.
TL;DR: A transport study of exfoliated few monolayer crystals of topological insulator Bi2Se3 in an electric field effect geometry finds that the temperature T and magnetic field dependent transport properties in the vicinity of this V(g) can be explained by a bulk channel with activation gap of approximately 50 meV and a relatively high-mobility metallic channel that dominates at low T.
Abstract: We report a transport study of exfoliated few monolayer crystals of topological insulator Bi2Se3 in an electric field effect geometry. By doping the bulk crystals with Ca, we are able to fabricate devices with sufficiently low bulk carrier density to change the sign of the Hall density with the gate voltage V(g). We find that the temperature T and magnetic field dependent transport properties in the vicinity of this V(g) can be explained by a bulk channel with activation gap of approximately 50 meV and a relatively high-mobility metallic channel that dominates at low T. The conductance (approximately 2×7e2/h), weak antilocalization, and metallic resistance-temperature profile of the latter lead us to identify it with the protected surface state. The relative smallness of the observed gap implies limitations for electric field effect topological insulator devices at room temperature.
TL;DR: PbSe was expected to have a smaller bandgap and higher thermalconductivity than PbTe, but these values are about the same at high temperature leading to comparable thermoelectric figure of merit, with zT> 1 achieved in heavily doped p-type PbSe.
Abstract: PbSe was expected to have a smaller bandgap and higher thermalconductivity than PbTe. Instead, these values are about the same at high temperature leading to comparable thermoelectric figure of merit, with zT> 1 achieved in heavily doped p-type PbSe.
TL;DR: In this article, the authors show that for the parent compound of the promising CIGS Cu(InxGa1−x)Se2 solar devices, local density and generalized gradient approximation (GGA) density functionals generally underestimate band gaps for semiconductors and sometimes incorrectly predict a metal.
Abstract: An essential issue in developing semiconductor devices for photovoltaics and thermoelectrics is to design materials with appropriate band gaps plus the proper positioning of dopant levels relative to the bands. Local density (LDA) and generalized gradient approximation (GGA) density functionals generally underestimate band gaps for semiconductors and sometimes incorrectly predict a metal. Hybrid functionals that include some exact Hartree−Fock exchange are known to be better. We show here for CuInSe2, the parent compound of the promising CIGS Cu(InxGa1−x)Se2 solar devices, that LDA and GGA obtain gaps of 0.0−0.01 eV (experiment is 1.04 eV), while the historically first global hybrid functional, B3PW91, is surprisingly better than B3LYP with band gaps of 1.07 and 0.95 eV, respectively. Furthermore, we show that for 27 related binary and ternary semiconductors, B3PW91 predicts gaps with a mean average deviation (MAD) of only 0.09 eV, which is substantially better than all modern hybrid functionals.
TL;DR: In this article, the compositional dependence of the physical properties of CZTSSe alloys through first-principles calculations was studied and it was shown that these mixed-anion alloys are highly miscible with low enthalpies of formation.
Abstract: A thin-film solar cell based on Cu${}_{2}$ZnSn(S,Se)${}_{4}$ (CZTSSe) alloy was recently found to exhibit a light to electricity conversion efficiency of $10%$, making it competitive with the more mature Cu(In,Ga)Se${}_{2}$ based technologies. We study the compositional dependence of the physical properties of CZTSSe alloys through first-principles calculations and find that these mixed-anion alloys are highly miscible with low enthalpies of formation, and the cations maintain the same ordering preferences as the parent compounds Cu${}_{2}$ZnSnS${}_{4}$ and Cu${}_{2}$ZnSnSe${}_{4}$. The band gap of the CZTSSe alloy decreases with the Se content almost linearly, and the band alignment between Cu${}_{2}$ZnSnS${}_{4}$ and Cu${}_{2}$ZnSnSe${}_{4}$ is of type I, which allows for more facile $n$-type and $p$-type doping for alloys with high Se content. Based on these results we analyze the influence of composition on the efficiency of CZTSSe solar cells and explain the high efficiency of the cells with high Se content.
TL;DR: In this paper, the authors demonstrate the dramatically different transport properties in trilayer graphene with different stacking orders, and the unexpected spontaneous gap opening in charge neutral r-TLG, which is a signature of the Lifshitz transition, a topological change in the Fermi surface.
Abstract: Graphene is an extraordinary two-dimensional (2D) system with chiral charge carriers and fascinating electronic, mechanical and thermal properties. In multilayer graphene, stacking order provides an important yet rarely explored degree of freedom for tuning its electronic properties. For instance, Bernal-stacked trilayer graphene (B-TLG) is semi-metallic with a tunable band overlap, and rhombohedral-stacked trilayer graphene (r-TLG) is predicted to be semiconducting with a tunable band gap. These multilayer graphenes are also expected to exhibit rich novel phenomena at low charge densities owing to enhanced electronic interactions and competing symmetries. Here we demonstrate the dramatically different transport properties in TLG with different stacking orders, and the unexpected spontaneous gap opening in charge neutral r-TLG. At the Dirac point, B-TLG remains metallic, whereas r-TLG becomes insulating with an intrinsic interaction-driven gap ~6 meV. In magnetic fields, well-developed quantum Hall (QH) plateaux in r-TLG split into three branches at higher fields. Such splitting is a signature of the Lifshitz transition, a topological change in the Fermi surface, that is found only in r-TLG. Our results underscore the rich interaction-induced phenomena in trilayer graphene with different stacking orders, and its potential towards electronic applications.
TL;DR: If the MEG efficiency can be further enhanced and charge separation and transport can be optimized within QD films, then QD solar cells can lead to third-generation solar energy conversion technologies.
Abstract: Multiple exciton generation in quantum dots (QDs) has been intensively studied as a way to enhance solar energy conversion by utilizing the excess energy in the absorbed photons. Among other useful properties, quantum confinement can both increase Coulomb interactions that drive the MEG process and decrease the electron–phonon coupling that cools hot excitons in bulk semiconductors. However, variations in the reported enhanced quantum yields (QYs) have led to disagreements over the role that quantum confinement plays. The enhanced yield of excitons per absorbed photon is deduced from a dynamical signature in the transient absorption or transient photoluminescence and is ascribed to the creation of biexcitons. Extraneous effects such as photocharging are partially responsible for the observed variations. When these extraneous effects are reduced, the MEG efficiency, defined in terms of the number of additional electron–hole pairs produced per additional band gap of photon excitation, is about two times bet...
TL;DR: In this paper, the electronic properties of hydrogenated silicene and germanene, so called silicane and Germanane, respectively, are investigated using first-principles calculations based on density functional theory.
Abstract: The electronic properties of hydrogenated silicene and germanene, so called silicane and germanane, respectively, are investigated using first-principles calculations based on density functional theory. Two different atomic configurations are found to be stable and energetically degenerate. Upon the adsorption of hydrogen, an energy gap opens in silicene and germanene. Their energy gaps are next computed using the HSE hybrid functional as well as the G0W0 many-body perturbation method. These materials are found to be wide band-gap semiconductors, the type of gap in silicane (direct or indirect) depending on its atomic configuration. Germanane is predicted to be a direct-gap material, independent of its atomic configuration, with an average energy gap of about 3.2 eV, this material thus being potentially interesting for optoelectronic applications in the blue/violet spectral range.
TL;DR: A three-dimensional, 70-nm-thick HgTe layer, which is strained by epitaxial growth on a CdTe substrate, exhibits a quantized Hall effect that results from the 2D single cone Dirac-like topological surface states.
Abstract: We report transport studies on a three-dimensional, 70-nm-thick HgTe layer, which is strained by epitaxial growth on a CdTe substrate. The strain induces a band gap in the otherwise semimetallic HgTe, which thus becomes a three-dimensional topological insulator. Contributions from residual bulk carriers to the transport properties of the gapped HgTe layer are negligible at mK temperatures. As a result, the sample exhibits a quantized Hall effect that results from the 2D single cone Dirac-like topological surface states.
TL;DR: The measured thermal conductivity of a series of single crystalline silicon PnCs is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal Conductivity.
Abstract: Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.
TL;DR: Observations suggest that plasmonic nanostructures (which can be fabricated with absorption properties that cover the full solar spectrum) can function as a viable alternative to organic photosensitizers for photovoltaic and photodetection applications.
Abstract: A fruitful paradigm in the development of low-cost and efficient photovoltaics is to dope or otherwise photosensitize wide band gap semiconductors in order to improve their light harvesting ability for light with sub-band-gap photon energies.1–8 Here, we report significant photosensitization of TiO2 due to the direct injection by quantum tunneling of hot electrons produced in the decay of localized surface-plasmon polaritons excited in gold nanoparticles (AuNPs) embedded in the semiconductor (TiO2). Surface plasmon decay produces electron–hole pairs in the gold.9–15 We propose that a significant fraction of these electrons tunnel into the semiconductor’s conduction band resulting in a significant electron current in the TiO2 even when the device is illuminated with light with photon energies well below the semiconductor’s band gap. Devices fabricated with (nonpercolating) multilayers of AuNPs in a TiO2 film produced over 1000-fold increase in photoconductance when illuminated at 600 nm over what TiO2 film...