Showing papers on "Band gap published in 2000"
TL;DR: In this article, the absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals, and the relationship between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions were reviewed.
Abstract: The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H2O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H2O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H2O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.
3,145 citations
TL;DR: In this paper, the Schottky barrier heights and band offsets for high dielectric constant oxides on Pt and Si were calculated and good agreement with experiment is found for barrier heights.
Abstract: Wide-band-gap oxides such as SrTiO3 are shown to be critical tests of theories of Schottky barrier heights based on metal-induced gap states and charge neutrality levels. This theory is reviewed and used to calculate the Schottky barrier heights and band offsets for many important high dielectric constant oxides on Pt and Si. Good agreement with experiment is found for barrier heights. The band offsets for electrons on Si are found to be small for many key oxides such as SrTiO3 and Ta2O5 which limit their utility as gate oxides in future silicon field effect transistors. The calculations are extended to screen other proposed oxides such as BaZrO3. ZrO2, HfO2, La2O3, Y2O3, HfSiO4, and ZrSiO4. Predictions are also given for barrier heights of the ferroelectric oxides Pb1−xZrxTiO3 and SrBi2Ta2O9 which are used in nonvolatile memories.
1,947 citations
TL;DR: In this paper, the authors present an approach to calculate electron-hole excitations and the optical spectra of condensed matter from first principles, which combines three computational techniques: the electronic ground state is treated within density-functional theory, the single-particle spectrum of the electrons and holes is obtained within the approximation to the electron self-energy operator, and the electron and hole interaction is calculated and a Bethe-Salpeter equation is solved.
Abstract: We present a recently developed approach to calculate electron-hole excitations and the optical spectra of condensed matter from first principles The key concept is to describe the excitations of the electronic system by the corresponding one- and two-particle Green's function The method combines three computational techniques First, the electronic ground state is treated within density-functional theory Second, the single-particle spectrum of the electrons and holes is obtained within the $\mathrm{GW}$ approximation to the electron self-energy operator Finally, the electron-hole interaction is calculated and a Bethe-Salpeter equation is solved, yielding the coupled electron-hole excitations The resulting solutions allow the calculation of the entire optical spectrum This holds both for bound excitonic states below the band gap, as well as for the resonant spectrum above the band gap We discuss a number of technical developments needed for the application of the method to real systems To illustrate the approach, we discuss the excitations and optical spectra of spatially isolated systems (atoms, molecules, and semiconductor clusters) and of extended, periodic crystals (semiconductors and insulators)
1,382 citations
TL;DR: In this article, β-Ga2O3 with an energy band gap of 4.9 eV was prepared on silica glass substrates by a pulsed-laser deposition method, and the resulting internal transmittance at the wavelength (248 nm) of the KrF excimer laser exceeded 50% for the 100-nm-thick film.
Abstract: Thin films of β-Ga2O3 with an energy band gap of 4.9 eV were prepared on silica glass substrates by a pulsed-laser deposition method. N-type conductivity up to ∼1 S cm−1 was obtained by Sn-ion doping and deposition under low O2 partial pressure (∼10−5 Pa) at substrate temperatures above 800 °C. The resulting internal transmittance at the wavelength (248 nm) of the KrF excimer laser exceeded 50% for the 100-nm-thick film, making this the most ultraviolet-transparent conductive oxide thin film to date and opening up prospects for applications such as ultraviolet transparent antistatic electric films in ultraviolet lithography.
797 citations
TL;DR: It is found that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.
Abstract: Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers; this is analogous to Hund's rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters, storage devices and fluorescence markers. Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.
782 citations
TL;DR: In this article, the authors used NiO as a cocatalyst for water splitting into H2 and O2 in pure water without any additives under UV irradiation, and found that NiO increased the photocatalytic activity of the NiO(0.15 wt %)/Sr2Ta2O7 photocatalyst.
Abstract: Sr2Ta2O7 and Sr2Nb2O7 with similar layered perovskite structure showed activities for water splitting into H2 and O2 in pure water without any additives under UV irradiation. The band gaps of Sr2Ta2O7 and Sr2Nb2O7 were 4.6 and 3.9 eV, respectively. Sr2Ta2O7 gave H2 and O2 from pure water under UV irradiation even in the absence of a cocatalyst. The activity of Sr2Ta2O7 was much increased by loading NiO as a cocatalyst even without pretreatment. The quantum yield of the NiO(0.15 wt %)/Sr2Ta2O7 photocatalyst was 12% at 270 nm. On the other hand, native Sr2Nb2O7 did not possess the activity. The high activity was obtained for the Sr2Nb2O7 photocatalyst when NiO was loaded and pretreated. Factors affecting the photocatalytic activities were discussed by using the Sr2Ta2O7 and Sr2Nb2O7 powder with similar layered perovskite structure. A predominant factor affecting the photocatalytic behavior of Sr2Ta2O7 and Sr2Nb2O7 is the conduction band levels formed by Ta5d and Nb4d.
583 citations
TL;DR: The theory unriddles and unifies previous band gap studies and predicts the shifting, merging, and splitting of Van Hove singularities in the density of state, and the zigzag pattern of band gap change with strains.
Abstract: Electronic structure of deformed carbon nanotubes varies widely depending on their chirality and deformation mode. We present a framework to analyze these variations by quantifying the dispersion relation and density of states. The theory is based on the H\"uckel tight-binding model and confirmed by four orbital tight-binding simulations of nanotubes under stretching, compression, torsion, and bending. It unriddles and unifies previous band gap studies and predicts the shifting, merging, and splitting of Van Hove singularities in the density of state, and the zigzag pattern of band gap change with strains. Possible applications to nanotube devices and spectroscopy research are also presented.
561 citations
TL;DR: In this article, electrical conductivity and optical properties of undoped zinc oxide films prepared by the sol-gel process using a spin-coating technique were investigated, and it was shown that the optical band gap energy for the films was 3.20-3.21 eV and the electronic transition was of the direct transition type.
454 citations
TL;DR: In this paper, capacitance and voltage measurements for metal-oxide-semiconductor capacitors fabricated using the 4H polytype of silicon carbide doped with either nitrogen (n) or aluminum (p).
Abstract: Results of capacitance–voltage measurements are reported for metal–oxide–semiconductor capacitors fabricated using the 4H polytype of silicon carbide doped with either nitrogen (n) or aluminum (p). Annealing in nitric oxide after a standard oxidation/reoxidation process results in a slight increase in the defect state density in the lower portion of the band gap for p-SiC and a significant decrease in the density of states in the upper half of the gap for n-SiC. Theoretical calculations provide an explanation for these results in terms of N passivating C and C clusters at the oxide–semiconductor interface.
453 citations
02 May 2000-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, a review of matter transformation induced in crystalline inorganic insulators by swift heavy ions is presented, with the emphasis on new results obtained for amorphizable materials such as Gd3Ga5O12, GeS, and LiNbO3 and for nonamorphizable crystals such as SnO2, LiF and CaF2.
Abstract: A review of matter transformation induced in crystalline inorganic insulators by swift heavy ions is presented. The emphasis is made on new results obtained for amorphizable materials such as Gd3Ga5O12, GeS, and LiNbO3 and for non-amorphizable crystals such as SnO2, LiF and CaF2. Assuming that latent tracks result from a transient thermal process, a quantitative development of a thermal spike is proposed. The only free parameter is the electron–lattice interaction mean free path λ. With this parameter it is possible to quantitatively describe track radii, whatever the bonding character of the crystal is, in a wide range of ion velocities assuming two specific criteria: tracks may result from a rapid quenching of a cylinder of matter in which the energy deposited on the lattice has overcome either the energy necessary to reach a quasi-molten phase in the case of amorphizable materials or the cohesion energy in the case of non-amorphizable materials. The evolution of the λ parameter versus the band gap energy of the considered insulator will be presented. On the basis of this discussion some predictions are developed.
425 citations
TL;DR: In this paper, the alloy bowing coefficients, alloy mixing enthalpies, and interfacial valence band offsets for three Cd-based (CdS, CdSe, and CdTe) compounds were calculated.
Abstract: Using first principles band structure theory we have calculated (i) the alloy bowing coefficients, (ii) the alloy mixing enthalpies, and (iii) the interfacial valence band offsets for three Cd-based (CdS, CdSe, CdTe) compounds. We have also calculated defect formation energies and defect transition energy levels of Cd vacancy VCd and CuCd substitutional defect in CdS and CdTe, as well as the isovalent defect TeS in CdS. The calculated results are compared with available experimental data.
TL;DR: In this paper, electrical and optical properties of CuAlO2, a p-type conducting transparent oxide, were examined for the thin films prepared by the pulsed laser deposition technique, and the indirect and direct allowed optical band gaps were evaluated to be ∼1.8 and ∼3.5 eV, respectively.
Abstract: Electrical and optical properties of CuAlO2, a p-type conducting transparent oxide, were examined for the thin films prepared by the pulsed laser deposition technique. The indirect and direct allowed optical band gaps were evaluated to be ∼1.8 and ∼3.5 eV, respectively. The conductivity at 300 K was ∼3×10−1 S cm−1 and its temperature dependence is of the thermal-activation type (activation energy ≈0.2 eV) at temperatures >220 K but is of the variable-range hopping type (log σ∝T−1/4) at <220 K. It was inferred that an admixed state of Cu 3d and O 2p primarily constitutes the upper valence band, which controls transport of positive holes, from a combined information on ultraviolet photoemission spectrum with x-ray photoemission spectrum. An energy band calculation by full-potential linearized augmented plane wave method substantiated the experimental findings. The present results gave a solid basis for our working hypothesis [Nature (London) 389, 939 (1997)] for chemical design of p-type conducting transpar...
TL;DR: The mechanism for energy transfer leading to electroluminescence (EL) of a lanthanide complex, Eu(TTA)3phen (TTA=thenoyltrifluoroacetone,phen=1,10-phenanthroline), doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host is investigated in this paper.
Abstract: The mechanism for energy transfer leading to electroluminescence (EL) of a lanthanide complex, Eu(TTA)3phen (TTA=thenoyltrifluoroacetone,phen=1,10-phenanthroline), doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host is investigated. With the device structure of anode/hole transport layer/Eu(TTA)3phen(1%):CPB/electron transport layer/cathode, we achieve a maximum external EL quantum efficiency (η) of 1.4% at a current density of 0.4 mA/cm2. Saturated red Eu3+ emission based on 5Dx–7Fx transitions is centered at a wavelength of 612 nm with a full width at half maximum of 3 nm. From analysis of the electroluminescent and photoluminescent spectra, and the current density–voltage characteristics, we conclude that direct trapping of holes and electrons and subsequent formation of the excitons occurs on the dopant, leading to high quantum efficiencies at low current densities. With increasing current between 1 and 100 mA/cm2, however, a significant decrease of η along with an increase in CBP host emission is observed. We demonstrate that the decrease in η at high current densities can be explained by triplet–triplet annihilation.
TL;DR: In this article, a liquid crystal was infiltrated into the air pores of a macroporous silicon photonic crystal with a triangular lattice pitch of 1.58 and a band gap wavelength range of 3.3-5.7 µm.
Abstract: The photonic band gap of a two-dimensional photonic crystal is continuously tuned using the temperature dependent refractive index of a liquid crystal. Liquid crystal $E7$ was infiltrated into the air pores of a macroporous silicon photonic crystal with a triangular lattice pitch of 1.58 $\ensuremath{\mu}$m and a band gap wavelength range of 3.3--5.7 \ensuremath{\mu}m. After infiltration, the band gap for the H polarized field shifted dramatically to 4.4--6.0 \ensuremath{\mu}m while that of the E-polarized field collapsed. As the sample was heated to the nematic-isotropic phase transition temperature of the liquid crystal $(59\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ the short-wavelength band edge of the H gap shifted by as much as 70 nm while the long-wavelength edge was constant within experimental error. Band structure calculations incorporating the temperature dependence of the liquid crystal birefringence can account for our results and also point to an escaped-radial alignment of the liquid crystal in the nematic phase.
TL;DR: The effect of oxygenation on the electronic properties of semiconducting carbon nanotubes is studied from first principles and weak hybridization between carbon and oxygen is predicted for the valence-band edge states.
Abstract: The effect of oxygenation on the electronic properties of semiconducting carbon nanotubes is studied from first principles. The O2 is found to bind to a single-walled nanotube with an adsorption energy of about 0.25 eV and to dope semiconducting nanotubes with hole carriers. Weak hybridization between carbon and oxygen is predicted for the valence-band edge states. The calculated density of states shows that weak coupling leads to conducting states near the band gap. The oxygen-induced gap closing for large-diameter semiconducting tubes is discussed as well. The influence of oxygen on the magnetic property is also addressed through a spin-polarized calculation and compared to experiment.
TL;DR: In this paper, the authors review the properties of heavy fermion semiconductors and show how they can be interpreted in terms of an electronic band structure, with a temperature dependent hybridization gap together with temperature dependent quasi-particle lifetimes.
Abstract: The heavy fermion semiconductors, or Kondo insulators, are very narrow gap semiconductors in which the properties show unusual temperature dependencies. We shall review their properties and show how they can be interpreted in terms of an electronic band structure, with a temperature dependent hybridization gap together with temperature dependent quasi-particle lifetimes. The properties of these semiconductors are very sensitive to impurities, which can enhance the incipient antiferromagnetic correlations and precipitate a magnetic instability.
TL;DR: In this article, the authors conducted a systematic investigation of solid solution thin films of Zn 1− x Mg x O, a group of ternary compounds of the Zn-Mg-O system.
TL;DR: In this article, the absorption properties of anatase TiO2 nanoparticles sensitized by catechol and benzoic acid are reported, and a direct photoinjection scheme which does not involve excited states of the adsorbate is proposed to account for the electron injection process.
Abstract: Results from quantum chemical INDO/S-CI calculations of the absorption properties of anatase TiO2 nanoparticles sensitized by catechol and benzoic acid are reported. Experimental observations that catechol causes a strong shift in the TiO2 absorption threshold, while benzoic acid does not, are explained from the calculations which indicate that only catechol introduces electronic levels in the TiO2 band gap. The 420 nm excitation in the TiO2 + catechol system responsible for the shift is seen to be a charge-transfer excitation from the catechol π HOMO orbital to Ti(3d) levels at the bottom of the conduction band. A direct photoinjection scheme which does not involve excited states of the adsorbate is proposed to account for the electron injection process.
TL;DR: In this paper, the first attempt at relating the size-induced transformation from a hexagonal to a cubic structure in CdS nanoparticles to a change in the band gap was made.
Abstract: The interrelation between particle size, crystal structure and optical properties in semiconductor quantum dots has elicited widespread interest. We report the first attempt at relating the size-induced transformation from a hexagonal to a cubic structure in CdS nanoparticles to a change in the band gap. CdS nanoparticles with particle size in the 0.7-10 nm range were prepared by chemical precipitation using thiophenol as a capping agent. Whereas the band gap for bulk hexagonal CdS is about 2.5 eV, that for 1 nm cubic CdS nanoparticles was found to be almost 3.9 eV. We also suggest a simple mechanism (based on the periodic insertion of stacking faults) for the transformation from the cubic zinc blende structure to the hexagonal wurtzite structure.
TL;DR: In this article, the authors adopt an atomistic pseudopotential description of the electronic structure of self-assembled, lens-shaped InAs quantum dots within the ''linear combination of bulk bands'' method.
Abstract: We adopt an atomistic pseudopotential description of the electronic structure of self-assembled, lens-shaped InAs quantum dots within the ``linear combination of bulk bands'' method. We present a detailed comparison with experiment, including quantites such as the single-particle electron and hole energy level spacings, the excitonic band gap, the electron-electron, hole-hole, and electron-hole Coulomb energies and the optical polarization anisotropy. We find a generally good agreement, which is improved even further for a dot composition where some Ga has diffused into the dots.
TL;DR: In this article, the authors derived the refractive indices of AlxGa1−xAs epitaxial layers from the modal propagation constants in the range of 730 nm <λ<830 nm with an estimated uncertainty of Δn=5×10−4.
Abstract: The refractive indices of AlxGa1−xAs epitaxial layers (0.176⩽x⩽1) are accurately determined below the band gap to wavelengths, λ<3 μm. The layers are grown on GaAs substrates by molecular beam epitaxy metal organic and chemical vapor deposition with thicknesses ranging from 4 to 10 μm. They form improper waveguide structures with the GaAs substrate. The measurements are based on the excitation of the improper waveguide modes with grating couplers at 23 °C. The refractive indices of the layers are derived from the modal propagation constants in the range of 730 nm<λ<830 nm with an estimated uncertainty of Δn=5×10−4. The temperature coefficient of the refractive index is investigated in the same spectral range. From the effective indices of the TE and TM modes, we derive the strain-induced birefringence and the elasto-optic coefficients. High-resolution x-ray diffraction is used to determine the strain of the layers. The layer compositions are obtained with inductively coupled plasma atomic emission spectro...
TL;DR: In this paper, the photoluminescence properties of surface-oxidized Si nanocrystals (nc-Si) were studied as a function of the size of the sample and a high energy shift of the peak from the vicinity of the bulk band gap to the visible region was observed.
Abstract: Photoluminescence (PL) from surface-oxidized Si nanocrystals (nc-Si) was studied as a function of the size. The size of nc-Si was comparable with or larger than the Bohr radius of free excitons in bulk Si crystal (5 nm). In contrast to smaller surface-oxidized nc-Si (typically as small as a few nanometers in diameter), these relatively large nc-Si exhibited PL properties with strong size dependence. A high-energy shift of the PL peak from the vicinity of the bulk band gap to the visible region was observed. This PL shift was accompanied by a shortening of the PL lifetime and an increase in the exchange splitting energy of excitons. These size dependences indicate that the PL originates from the recombination of excitons confined in nc-Si. The differences in the PL properties between H-terminated and surface-oxidized nc-Si are also discussed.
TL;DR: Electronic transport measurements were performed on Y-junction carbon nanotubes, which represent new heterojunctions for nanoelectronics and show intrinsic nonlinear transport and reproducible rectifying behavior at room temperature.
Abstract: Electronic transport measurements were performed on Y-junction carbon nanotubes. These novel junctions contain a large diameter tube branched into smaller ones. Independent measurements using good quality contacts on both individual Y junctions and many in parallel show intrinsic nonlinear transport and reproducible rectifying behavior at room temperature. The results were modeled using classic interface physics for a junction with an abrupt change in band gap due to the change in tube diameter. These Y-junction tubes represent new heterojunctions for nanoelectronics.
TL;DR: In this article, a transparent p-type semiconductor for optoelectronic applications, La1−xSrxCuOS, was shown to have high optical transmission at the visible and near-infrared wavelengths and an energy gap of 3.1 eV.
Abstract: La1−xSrxCuOS (x=0, 0.05) thin films prepared by radio-frequency sputtering were found to have high optical transmission (⩾70%) at the visible and near-infrared wavelengths and an energy gap of about 3.1 eV. The dc electrical conductivities of x=0 and 0.05 thin films at room temperature were 1.2×10−2 and 2.6×10−1 S cm−1, respectively. The Seebeck coefficients of these samples were positive, indicating that p-type electrical conduction is dominant in these materials. A sharp photoluminescence peak, probably originating from an interband transition, was observed at the optical absorption edge. The present study demonstrates that LaCuOS is a promising transparent p-type semiconductor for optoelectronic applications. Moreover, our material design, based on chemical modulation of the valence band, was successfully extended to oxysulfide systems.
01 Mar 2000
TL;DR: In this paper, the effect of oxygenation on the electronic properties of semiconducting carbon nanotubes was studied from first principles, and the O2 was found to bind to a single-walled nanotube with an adsorption energy of about 0.25 eV.
Abstract: The effect of oxygenation on the electronic properties of semiconducting carbon nanotubes is studied from first principles. The O2 is found to bind to a single-walled nanotube with an adsorption energy of about 0.25 eV and to dope semiconducting nanotubes with hole carriers. Weak hybridization between carbon and oxygen is predicted for the valence-band edge states. The calculated density of states shows that weak coupling leads to conducting states near the band gap. The oxygen-induced gap closing for large-diameter semiconducting tubes is discussed as well. The influence of oxygen on the magnetic property is also addressed through a spin-polarized calculation and compared to experiment.
TL;DR: In this paper, transient and persistent diffuse reflectance infrared signals from surface intermediates generated by band gap irradiation of polycrystalline, hydroxylated TiO_2 under controlled atmospheres were detected.
Abstract: We detect transient and persistent diffuse reflectance infrared signals from surface intermediates generated by band gap irradiation of polycrystalline, hydroxylated TiO_2 under controlled atmospheres. Irradiation in vacuo or in the presence of CD_3OD(ads) leads to a new absorbance at 3716 cm^(-1), the decline of a 3647 cm^(-1) band, and a reflectivity loss. Overall reflectivity, which gauges mobile charge, partially recovers in the dark after a few minutes but fully recovers upon exposure to O_2. The 3716 cm^(-1) feature persists for days in a vacuum or dry O_2 but is bleached by the action of (O_2 + H_2O + hν). It is assigned to a Ti(III)O−H^- vibration resulting from electron trapping at acidic Ti(IV)OH centers. Irradiation under O_2 yields a new band at 3683 cm^(-1), ascribed to surface-bound OH radicals that last indefinitely in a vacuum. None of the above events occur on thermally generated TiO_(2-γ) specimens subsequently exposed to H_2O(g). We infer that (1) O_2 is an efficient scavenger of conduction band electrons at the gas−solid interface, (2) the competition for holes between lattice oxygen photodesorption and OH^- oxidation depends on gas pressure and composition, and (3) the trapped carrier buildup eventually results in extended surface reconstruction involving Ti−OH functionalities, giving rise to restructuring hysteresis.
TL;DR: In this paper, the general aspects of the electronic structure of the XTZ semi-Heusler systems are discussed using the Korringa-Kohn-Rostoker computations within the LDA framework.
TL;DR: In this paper, two transitions in the bonding are found in tetrahedral amorphous carbon (ta-C) films as a function of deposition temperature, and the sp2 sites show a gradual ordering into the graphitic clusters through the sharp bonding transition.
Abstract: Two transitions in the bonding are found in tetrahedral amorphous carbon (ta-C) films as a function of deposition temperature. The total sp3 fraction shows a sharp decrease at a transition temperature of order 250 °C. In contrast, visible Raman finds that the sp2 sites show a gradual ordering into the graphitic clusters through the sharp bonding transition. The optical gap and resistivity show a similar, gradual transition. This indicates that the sp2 cluster size determines the optical gap, even when the sp2 content does not change. The Raman I(D)/I(G) peak ratio is found to vary inversely with the square of the gap.
TL;DR: In this paper, a guest-free clathrate form of crystalline silicon was achieved by successive vacuum treatment and density separation of ${\mathrm{Na}}{x}{\mathm{Si}}_{136}$-based materials.
Abstract: Synthesis of a guest-free clathrate form of crystalline silicon was achieved by successive vacuum treatment and density separation of ${\mathrm{Na}}_{x}{\mathrm{Si}}_{136}$-based materials. The new allotrope of silicon has an open framework structure based upon slightly distorted tetrahedral atoms bound into five- and six-membered ring structures, and corresponds to a fully saturated and condensed fullerane-type solid. Theoretical calculations indicate that the new form of silicon should be a wide bandgap semiconductor. This prediction is borne out by experiment: electrical conductivity and optical absorption measurements yield a band gap of 1.9 eV, approximately twice the value of ``normal'' semiconducting silicon.