Small Band Gap Semiconductors Research Articles

Charge disproportionation in tetragonal La2MoO5, a small band gap semiconductor influenced by direct Mo-Mo bonding.

The structure of the novel compound La2MoO5 has been solved from powder X-ray and neutron diffraction data and belongs to the tetragonal space group P4/m (no. 83) with a = 12.6847(3) Å and c = 6.0568(2) Å and with Z = 8. It consists of equal proportions of bioctahedral (Mo2O10) and square prismatic (Mo2O8) dimers, both of which contain direct Mo-Mo bonds and are arranged in 1D chains. The Mo-Mo bond length in the Mo2O10 dimers is 2.684(8) Å, while there are two types of Mo2O8 dimers with Mo-Mo bonds lengths of 2.22(2) and 2.28(2) Å. Although the average Mo oxidation state in La2MoO5 is 4+, the very different Mo-Mo distances reflect the fact that the Mo2O10 dimers contain only Mo(5+) (d(1)), while the prismatic Mo2O8 dimers only contain Mo(3+) (d(3)), a result directly confirmed by density function theory calculations. This is due to the complete disproportionation of Mo(4+), a phenomenon which has not previously been observed in solid-state compounds. La2MoO5 is diamagnetic, behavior which is not expected for a nonmetallic transition-metal oxide whose cation sites have an odd number of d-electrons. The resistivity displays the Arrhenius-type activated behavior expected for a semiconductor with a band gap of 0.5 eV, exhibiting an unusually small transport gap relative to other diamagnetic oxides. Diffuse reflectance studies indicate that La2MoO5 is a rare example of a stable oxide semiconductor with strong infrared absorbance. It is shown that the d-orbital splitting associated with the Mo2O8 and Mo2O10 dimeric units can be rationalized using simple molecular orbital bonding concepts.

Nanostructured Photocathodes for Infrared Photodetectors and Photovoltaics

Interfacial energy band alignment is crucial for applications involving charge transfer and transport. Specifically, efficient photovoltaic (PV) devices require fine-tuning of the energy levels at interfaces between the absorber and the electrodes for extraction of photogenerated charges. Infrared (IR) absorbers have a small bandgap, hence such tuning is difficult using common electrode materials. Semiconductor sensitized PV devices typically utilize transparent metal oxide photoanodes, since many semiconductor absorbers can inject photoexcited electrons into their conduction bands. Transparent photocathode electrodes, to which photoexcited holes are transferred from semiconductor absorbers, are less common. Transparent nanostructured photocathodes with beneficial energy level alignment with small bandgap semiconductors can widen the material choices for IR photoelectrodes. Herein we show that CuSCN nanowire (NW) arrays can serve as such photocathodes, with efficient transfer of photogenerated holes from bulk-like PbS absorbing in the IR and near-IR range. Pulsed electrodeposition was used to fabricate high-quality CuSCN NW arrays. Significant NIR and IR photocurrents showed that the CuSCN NW array is an efficient photocathode for PbS sensitized IR photovoltaic devices. Comparison of various electrode materials verified the photogenerated hole injection vs competing processes such as electron injection and recombination and suggested that this material system can be used for studying photosensitized hole injection from other small bandgap semiconductors.

Influence of lattice expansion on the topological band order of InAsxSb1−x (x = 0, 0.25, 0.5, 0.75, 1) alloys

The topological band structures of InAsxSb1−x (x=0, 0.25, 0.5, 0.75, 1) alloys have been investigated using density functional theory by utilizing the Wien2k package. These alloys are in a topologically trivial phase in their unstrained states and exhibit a small band gap. Since in small band-gap cubic semiconductors the nontrivial topological phase can be achieved by lattice expansion, we investigate the effect of hydrostatic and biaxial lattice expansion on band inversion strength and band order of these alloys. It is found that under reasonable hydrostatic lattice expansion, InAsxSb1−x (x=0, 0.25, 0.75, 1) alloys with cubic symmetry and InAs0.5Sb0.5 alloy with tetragonal symmetry, are converted to nontrivial topological semiconductors with zero band gap and non-zero band gap, respectively. In order to convert InAsxSb1−x (x=0, 0.25, 0.75, 1) alloys into topological semiconductors with non-zero band gap, we let the systems of InAsxSb1−x (x=0, 0.25, 0.75, 1) alloys undergo a biaxial lattice expansion. Thus by breaking the cubic symmetry in these alloys, not only they are converted to topologically nontrivial phase but also a small band gap is opened at Γ point.

Granularity controlled nonsaturating linear magnetoresistance in topological insulator Bi2Te3 films.

We report on the magnetotransport properties of chemical vapor deposition grown films of interconnected Bi2Te3 nanoplates. Similar to many other topological insulator (TI) materials, these granular Bi2Te3 films exhibit a linear magnetoresistance (LMR) effect which has received much recent attention. Studying samples with different degree of granularity, we find a universal correlation between the magnitude of the LMR and the average mobility (⟨μ⟩) of the films over nearly 2 orders of magnitude change of ⟨μ⟩. The granularity controlled LMR effect here is attributed to the mobility fluctuation induced classical LMR according to the Parish-Littlewood theory. These findings have implications to both the fundamental understanding and magnetoresistive device applications of TI and small bandgap semiconductor materials.

First-principles investigation of the electronic and lattice vibrational properties of Mg2C

Mg2C, a newly synthesized magnesium carbide under high pressure, is a small band gap semiconductor. A first-principles investigation of the electronic and lattice dynamic properties of Mg2C are presented. We find the electronic band gap of Mg2C decreases slowly with pressure increasing. There are two optical vibrational modes at the Γ point of Mg2C. One is F1u mode, and the other is F2g mode. The former mode is infrared active and the latter one is Raman active. Their vibrational frequencies increase sharply with pressure increasing. Furthermore, we find that Mg2C has large infrared intensity and Raman activity, which indicates that infrared and Raman spectra may play a key role in identifying Mg2C. Moreover, we calculate the dielectric tensor and LO–TO splitting under different pressure and find large LO–TO splitting in the F1u mode of Mg2C. Our calculated results provide extraordinary insights into the infrared and Raman spectra of Mg2C.

First-principle study of nanostructures of functionalized graphene

We present first-principle calculations of 2D nanostructures of graphene functionalized with hydrogen and fluorine, respectively, in chair conformation. The partial density of states, band structure, binding energy and transverse displacement of C atoms due to functionalization (buckling) have been calculated within the framework of density functional theory as implemented in the SIESTA code. The variation in band gap and binding energy per add atom have been plotted against the number of add atoms, as the number of add atoms are incremented one by one. In all, 37 nanostructures with 18C atoms, 3 × 3 × 1 (i.e., the unit cell is repeated three times along x-axis and three times along y-axis) supercell, have been studied. The variation in C–C, C–H and C–F bond lengths and transverse displacement of C atoms (due to increase in add atoms) have been tabulated. A large amount of buckling is observed in the carbon lattice, 0.0053–0.7487 A, due to hydrogenation and 0.0002–0.5379 A, due to fluorination. As the number of add atoms (hydrogen or fluorine) is increased, a variation in the band gap is observed around the Fermi energy, resulting in change in behaviour of nanostructure from conductor to semiconductor/insulator. The binding energy per add atom increases with the increase in the number of add atoms. The nanostructures with 18C+18H and 18C+18F have maximum band gap of 4.98 eV and 3.64 eV, respectively, and binding energy per add atom –3.7562 eV and –3.3507 eV, respectively. Thus, these nanostructures are stable and are wide band-gap semiconductors, whereas the nanostructures with 18C+2H, 18C+4H, 18C+4F, 18C+8F, 18C+10F and 18C+10H atoms are small band-gap semiconductors with the band gap lying between 0.14 eV and 1.72 eV. Fluorine being more electronegative than hydrogen, the impact of electronegativity on band gap, binding energy and bond length is visible. It is also clear that it is possible to tune the electronic properties of functionalized graphene, which makes it a suitable material in microelectronics.

Three series of quaternary rare-earth transition-metal pnictides with CaAl2Si2-type structures: RECuZnAs2, REAgZnP2, and REAgZnAs2

Three series of quaternary rare-earth transition-metal pnictides REMM′Pn2 (M=Cu, Ag; M′=Zn; Pn=P, As) have been prepared by reaction of the elements at 800°C, with crystal growth promoted through the addition of iodine. The extent of RE substitution is broad in these series: RECuZnAs2 (RE=Y, LaNd, Sm, Gd–Lu), REAgZnP2 (RE=La–Nd, Sm, Gd–Dy), and REAgZnAs2 (RE=LaNd, Sm, GdDy). Powder and single-crystal X-ray diffraction analysis revealed that they adopt the trigonal CaAl2Si2-type structure (space group P3¯m1, Z=1), in which Cu or Ag atoms are disordered with Zn atoms over the unique tetrahedrally coordinated transition-metal site. Magnetic measurements indicated Curie–Weiss behavior for several members of the RECuZnAs2 and REAgZnP2 series. Core-line X-ray photoelectron spectra (XPS) collected on some RECuZnAs2 members corroborate the charge assignment deduced by the Zintl concept for these compounds, (RE3+)(M1+)(Zn2+)(Pn3−)2. Optical diffuse reflectance spectra and valence band XPS spectra established that these compounds are small band-gap semiconductors (up to ~0.8eV in REAgZnP2) or semimetals (RECuZnAs2). Band structure calculations also support this electronic structure and indicate that the band gap can be narrowed through appropriate chemical substitution (RE=smaller atoms, M=Cu, and Pn=As).

Electronic Structure of Ternary Rhodium Hydrides with Lithium and Magnesium

Chemical bonding in and electronic structure of lithium and magnesium rhodium hydrides are studied theoretically using DFT methods. For Li3RhH4 with planar complex RhH4 structural units, Crystal Orbital Hamilton Populations reveal significant Rh−Rh interactions within infinite one-dimensional ∞ 1 [RhH4] stacks in addition to strong rhodium−hydrogen bonding. These metal−metal interactions are considerably weaker in the hypothetical, heavier homologue Na3RhH4. Both compounds are small-band gap semiconductors. The electronic structures of Li3RhH6 and Na3RhH6 with rhodium surrounded octahedrally by hydrogen, on the other hand, are compatible with a classical complex hydride model according to the limiting ionic formula (M+)3[RhH6]3− without any metal−metal interaction between the 18-electron hydridorhodate complexes. In MgRhH, building blocks of the composition (RhH2)4 are formed with strong rhodium−hydrogen and significant rhodium−rhodium bonding (bond lengths of 298 pm within Rh4 squares). These units are linked together to infinite two-dimensional layers ∞ 2 [(RhH2/2)4] via common hydrogen atoms. Li3RhH4 and MgRhH are accordingly examples for border cases of chemical bonding where the classical picture of hydridometalate complexes in complex hydrides is not sufficient to properly describe the chemical bonding situation.

Enhanced Visible Light Photocatalysis of Bi2O3 upon Fluorination

For the better utilization of solar light and complete oxidation of environmental organic pollutants, it is desired to develop small band gap semiconductors with a deep valence band as efficient visible light photocatalysts. In this work, we prepared the fluorinated Bi2O3 catalysts using a precipitation method, followed by a solvothermal process in the presence of NH4F. The fluorinated Bi2O3 catalysts, especially with the atomic ratio of F to Bi (RF) at 0.2, exhibit much higher photocatalytic activities than the pure Bi2O3 for the degradation of methyl orange (MO) under the visible light irradiation. The effects of the fluorination on the phase structure, special surface areas, morphologies, optical properties, surface-adsorbed species, and electronic band structure of the Bi2O3 were investigated in detail. It was revealed that both the surface-adsorbed and lattice-substituted fluorine, induced by the fluorination to Bi2O3, play critical roles in the enhanced photocatalytic performance of the fluorinated Bi2O3. The two types of fluorine species effectively inhibit the recombination of the photoexcited electron–hole pairs by withdrawing the photoexcited electrons and increase the oxidation power of the photoexcited hole by lowering the valence band edge, respectively.

Designed Synthesis, Structure, and Properties of a Family of Ferecrystalline Compounds [(PbSe)1.00]m(MoSe2)n

The targeted synthesis of multiple compounds with specific controlled nanostructures and identical composition is a grand challenge in materials chemistry. We report the synthesis of the new metastable compounds [(PbSe)1.00]m(MoSe2)n using precursors each designed to self-assemble into a specific compound. To form a compound with specific values for m and n, the number of atoms within each deposited elemental layer was carefully controlled to provide the correct absolute number of atoms to form complete layers of each component structural unit. On low-temperature annealing, these structures self-assemble with a specific crystallographic orientation between the component structural units with atomically abrupt interfaces. There is rotational disorder between the component structural units and between MoSe2 basal plane units within the MoSe2 layers themselves. The lead selenide constituent has a distorted rock salt structure exactly m bilayers thick leading to peaks in the off-axis diffraction pattern as a result of the finite size of and rotational disorder between the crystallites. The in-plane lattice parameters of the PbSe and MoSe2 components are independent of the value of m and n, suggesting little or no strain caused by the interface between them. These compounds are small band gap semiconductors with carrier properties dominated by defects and exhibit extremely low thermal conductivity as a result of the rotational disorder. The thermal conductivity can be tuned by varying the ratio of the number of ordered PbSe rock salt layers relative to the number of rotationally disordered MoSe2 layers. This approach, based on controlling the local composition of the precursor and low temperature to limit diffusion rates, provides a general route to the synthesis of new compounds containing alternating layers of constituents with designed nanoarchitecture.

ChemInform Abstract: Structural and Optoelectronic Characterization of RF Sputtered ZnSnN2.

AbstractThin films of stoichiometric ZnSnN2, a new earth‐abundant small bandgap semiconductor, are prepared by reactive RF magnetron sputter deposition from Zn0.75Sn0.25 or from elemental Zn and Sn at about 250 °C in an atmosphere of Ar and N2.

Structural and vibrational stability of M and Z phases of silicon and germanium from first principles

First-principles calculations were performed to investigate the structural feasibility of M and Z phases (novel monoclinic and orthorhombic structures recently reported for carbon) for silicon and germanium. The lattice parameters, bulk modulus, vibrational properties, and elastic constants are calculated using the local density approximation to describe the exchange-correlation energy, while the optical properties are calculated by using Many-Body Perturbation Theory in the G0W0 approximation. Our results indicate that silicon and germanium with the proposed crystal symmetries are elastically and vibrationally stable and are small band-gap semiconductors. We discuss the possible synthesis of such materials.

Searching for low-workfunction phases in the Cs-Te system: The case of Cs2Te5

We have computationally explored workfunction values of Cs2Te5 surfaces, an existing crystalline phase of the Cs-Te system and a small bandgap semiconductor, in order to search for reduced workfunction alternatives of Cs2Te that preserve the exceptionally high quantum efficiency of the Cs2Te seasoned photoemissive material. We have found that the Cs2Te5(010) surface exhibits a workfunction value of ≈1.9 eV when it is covered by Cs atoms. Cs2Te5 is analogous to our recently proposed low-workfunction materials, Cs2TeC2, and other ternary acetylides [J. Z. Terdik et al., Phys. Rev. B 86, 035142 (2012)], in as much as it also contains quasi one-dimensional substructures embedded in a Cs-matrix, forming the foundation for anomalous workfunction anisotropy and low workfunction values. The one-dimensional substructures in Cs2Te5 are polytelluride ions in a tetragonal rod-like packing. Cs2Te5 has the advantage of simpler composition and availability as compared to Cs2TeC2; however, its low workfunction surface is less energetically favored to the other surfaces than in Cs2TeC2. A significant and remarkable advantage of Cs2Te5 as compared to Cs2Te is its high optical absorption of visible photons that can allow for high quantum efficiency electron emission at visible photon energies.

Phase Characterization, Thermal Stability, High-Temperature Transport Properties, and Electronic Structure of Rare-Earth Zintl Phosphides Eu3M2P4 (M = Ga, In)

Two rare-earth-containing ternary phosphides, Eu3Ga2P4 and Eu3In2P4, were synthesized by a two-step solid-state method with stoichiometric amounts of the constitutional elements. Refinements of the powder X-ray diffraction are consistent with the reported single-crystal structure with space group C2/c for Eu3Ga2P4 and Pnnm for Eu3In2P4. Thermal gravimetry and differential scanning calorimetry (TG-DSC) measurements reveal high thermal stability up to 1273 K. Thermal diffusivity measurements from room temperature to 800 K demonstrate thermal conductivity as low as 0.6 W/m·K for both compounds. Seebeck coefficient measurements from room temperature to 800 K indicate that both compounds are small band gap semiconductors. Eu3Ga2P4 shows p-type conductivity and Eu3In2P4 p-type conductivity in the temperature range 300-700 K and n-type conductivity above 700 K. Electronic structure calculations result in band gaps of 0.60 and 0.29 eV for Eu3Ga2P4 and Eu3In2P4, respectively. As expected for a valence precise Zintl phase, electrical resistivity is large, approximately 2600 and 560 mΩ·cm for Eu3Ga2P4 and Eu3In2P4 at room temperature, respectively. Measurements of transport properties suggest that these Zintl phosphides have potential for being good high-temperature thermoelectric materials with optimization of the charge carrier concentration by appropriate extrinsic dopants.

Enhanced ferromagnetism by adding electrons in triple-decker Gd–phthalocyanine

The electronic structures and magnetic properties of triple-decker Gd–phthalocyanine have been studied by using the HSE06 hybrid function method. The results show that triple-decker Gd–phthalocyanine is a small band gap semiconductor. Ferromagnetic coupling is unstable in neutral triple-decker Gd–phthalocyanine due to the small energy difference between the ferromagnetic and antiferromagnetic state. Additional electrons can enhance the energy difference; the largest for triple-decker Gd–phthalocyanine is 0.062 eV with two additional electrons per phthalocyanine ring. A larger energy difference can result in stable ferromagnetic coupling in triple-decker Gd–phthalocyanine. Analysis of the wave function distribution and electron density difference shows that the additional electrons will occupy the innermost conjugate ring. The innermost conjugate ring is filled by the additional two electrons per phthalocyanine ring. The more electrons that occupy the innermost conjugate ring, the more enhanced the pz–f exchange interaction is, which is the reason for the ferromagnetic coupling in triple-decker Gd–phthalocyanine. Thus, the ferromagnetic coupling of triple-decker Gd–phthalocyanine with two additional electrons per phthalocyanine ring is the most stable. The ferromagnetism of triple-decker Gd–phthalocyanine that is enhanced by the additional electrons has potential applications in organic spintronics.

On the Thermoelectric Properties of Zintl Compounds Mg3Bi2−x Pn x (Pn = P and Sb)

A series of Zintl compounds Mg3Bi2-xPnx (Pn = P and Sb) have been synthesized by the solid-state reaction method. While Sb can be substituted to a level as high as x = 1.0, P can be substituted only up to x = 0.5. The thermoelectric potential of these compounds has been evaluated by measuring resistivity (ρ), Seebeck (α) and Hall coefficients, and thermal conductivity between 80 K and 850 K. The measured resistivity and Seebeck coefficient values are consistent with those expected for small-bandgap semiconductors. Hall measurements suggest that the carriers are p type with concentration (p) increasing from ~1019 cm−3 to ~1020 cm−3 as the Bi content is increased. The Hall mobility decreases with increasing temperature (T) and reaches a more or less similar value (~45 cm2/V s) for all substituted compositions at room temperature. Due to mass defect scattering, the lattice thermal conductivity (κL) is decreased to a minimum of ~1.2 W/m K in Mg3BiSb. The power factor (α2/ρ) is found to be rather low and falls in the range 0.38 mW/m K2 to 0.66 mW/m K2. As expected, at a high temperature of 825 K, the total thermal conductivity (κ) of Mg3BiSb reaches an impressive value of ~1.0 W/m K. The highest dimensionless figure of merit (ZT) is realized for Mg3BiSb and is ~0.4 at 825 K.

A new spin on spintronics

By harnessing the way charge carriers move in a magnetic field, computing blocks based on semiconductor junctions have been made that are reconfigurable and can be interconnected to perform complex logic functions. See Letter p.72 Modern electronic devices demand ever-faster information processors and increased functionality. Logic devices based on magnetic effects could meet some of these needs. In particular, functionality is increased when both voltage and magnetic field can be used to control the device. Jinki Hong and colleagues have designed a microchannel, made from the small-bandgap semiconductor indium antimonide, with current–voltage characteristics that are strongly dependent on the sign and magnitude of an applied magnetic field. Circuits made from these devices can be programmed — and reprogrammed — to perform specific logic functions such as AND, OR, NAND and NOR. This approach could provide an interesting new platform for non-volatile reconfigurable logic circuits, in which the distinction between hardware and software virtually disappears.

Electronic and magnetism properties of half-bare zigzag silicon carbon nanoribbons from hybrid density functional calculations

Electronic and magnetic properties of half-bare zigzag silicon carbon nanoribbons (ZSiCNRs) have been studied by using HSE06 hybrid function. The calculation results show that the ZSiCNRs with bare Si edge and terminated C edge are metal with 1.3μB magnetic moments per supercell. While the half-bare ZSiCNRs with bare C edge are small band gap semiconductor with 2μB magnetic moments per supercell. Stable magnetism of the half-bare ZSiCNRs can be demonstrated by the total energy calculations of different spin configurations. It is found that the magnetic coupling of half-bare ZSiCNRs depends on the bare edge atoms, regardless of the terminated atoms (H or F). Our results indicate that the half-bare ZSiCNRs with turnable electronic and magnetic properties have potential applications in spintronic nanodevices.

Crystal structure and chemistry of topological insulators

Topological surface states, a new kind of electronic state of matter, have recently been observed on the cleaved surfaces of crystals of a handful of small band gap semiconductors. The underlying chemical factors that enable these states are crystal symmetry, the presence of strong spin orbit coupling, and an inversion of the energies of the bulk electronic states that normally contribute to the valence and conduction bands. The goals of this review are to briefly introduce the physics of topological insulators to a chemical audience and to describe the chemistry, defect chemistry, and crystal structures of the compounds in this emergent field.

Multi-layered mesoporous TiO2thin films with large pores and highly crystalline frameworks for efficient photoelectrochemical conversion

Mesoporous thin films with various compositions are unique architectures for photoelectrochemical (PEC) solar cells. In this paper, we report the synthesis of highly ordered, multi-layered, continuous mesoporous TiO2 thin films with uniform large pores, crystalline walls and tunable film thickness, via a ligand-assisted evaporation induced self assembly (EISA) method. A Ti(acetylacetone) precursor and a diblock copolymer PEO-b-PS are employed for the controlled assembly of the TiO2/template mesostructure, followed by a two-step pyrolysis that generates carbon residue as an intermediate protection layer to support the TiO2 framework and mesostructures during the crystallization. Other transition metal ion dopants (such as Cr, Ni and Co) can be facilely incorporated into the TiO2 frameworks by co-assembly of these metal acetylacetone precursors during the EISA process. The obtained TiO2 thin film possesses an ordered monoclinic mesostructure distorted from a (110)-oriented primitive cubic structure, uniform and tunable large pores of 10–30 nm, a large surface area of ∼100 m2 g−1 and a high crystallinity anatase wall. The film thickness can be well controlled from 150 nm to several microns to tune the absorption, with the capability of generating free-standing film morphologies. Furthermore, this designed architecture allows for effective post-deposition of other small-bandgap semiconductor nanomaterials inside the large, open and interconnecting mesopores, leading to significantly improved solar absorption and photoconversion. As a proof-of-concept, we demonstrate that the photoanodes made of 4.75 μm thick mesoporous TiO2 film with deposited cadmium sulfide quantum dots exhibit excellent performance in PEC water splitting, with an optimized photocurrent density of 6.03 mA cm−2 and a photoconversion efficiency of 3.9%. These multi-layered mesoporous TiO2-based thin films can serve as a unique architecture for PEC and other solar energy conversion and utilization.