Small Band Gap Semiconductors Research Articles

Pressure-induced Anderson-Mott transition in elemental tellurium

Elemental tellurium is a small band-gap semiconductor, which is always p-doped due to the natural occurrence of vacancies. Its chiral non-centrosymmetric structure, characterized by helical chains arranged in a triangular lattice, and the presence of a spin-polarized Fermi surface, render tellurium a promising candidate for future applications. Here, we use a theoretical framework, appropriate for describing the corrections to conductivity from quantum interference effects, to show that a high-quality tellurium single crystal undergoes a quantum phase transition at low temperatures from an Anderson insulator to a correlated disordered metal at around 17 kbar. Such insulator-to-metal transition manifests itself in all measured physical quantities and their critical exponents are consistent with a scenario in which a pressure-induced Lifshitz transition shifts the Fermi level below the mobility edge, paving the way for a genuine Anderson-Mott transition. We conclude that previously puzzling quantum oscillation and transport measurements might be explained by a possible Anderson-Mott ground state and the observed phase transition.

Charge Transfer and Recombination Processes at p-CuBi2O4 Photoelectrodes

Photoelectrochemical water splitting is an attractive method to convert solar energy to storable chemical energy in the form of hydrogen, however, the materials requirements to achieve this efficiently are challenging: the semiconducting material needs to absorb sunlight efficiently, must to be capable of reducing and/or oxidizing water, and has to be stable under illumination under current flow in an aqueous electrolyte solution. In particular, for small bandgap semiconductors, stability is often an issue, and it is difficult to fully avoid degradation of the material. In addition, the water reduction and oxidation processes need to occur fast, in order to favorably compete with recombination or surface degradation processes; hence, the kinetic rate constants for charge transfer and surface recombination are very important parameters. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful technique to study the carrier dynamics in a photoelectrochemical cell. The photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge transfer and surface recombination can be determined, provided a simple model can be applied.1 In this presentation, we focus on the dynamic properties of p-CuBi2O4 in order to elucidate the rate determining steps that determine the efficiency of photocathodic reactions. In inert aqueous solutions at pH 5, an unfavorable balance exists between the rate constants for charge transfer and surface recombination, which limits the conversion efficiency.2 On the other hand, upon adding H2O2 as an electron acceptor, the photoelectron transfer efficiency is improved related to faster electron transfer to the solution or slower surface recombination due to passivation effects. Strategies to improve the applicability of CuBi2O4 in solar water splitting systems are discussed.The authors gratefully acknowledge CONACYT, SENER and CICY for funding through the Renewable Energy Laboratory of South East Mexico (LENERSE; Project 254667; SP-4), and CONACYT under the Basic Sciences (CB) project A1-S-28734. References “Photoelectrochemical Water Splitting at Semiconductor Electrodes: Fundamental Problems and New Perspectives”. L. M. Peter and K. G. Upul Wijayantha, ChemPhysChem, 15, 1983–1995 (2014). “Charge Transfer and Recombination Dynamics at Inkjet-Printed CuBi2O4 Electrodes for Photoelectrochemical Water Splitting”. Ingrid Rodríguez-Gutiérrez, Rodrigo García-Rodríguez, Manuel Rodríguez-Pérez, Alberto Vega-Poot, Geonel Rodríguez Gattorno, Bruce A. Parkinson, and Gerko Oskam. J. Phys. Chem. C, 122, 27169−27179 (2018).

Climbing Jacob’s ladder: A density functional theory case study for Ag2ZnSnSe4 and Cu2ZnSnSe4

Recent advances in the development of exchange and correlation functionals to be employed in density functional theory calculations combined with the availability of ever more powerful high-performance computing facilities, let predictive computational materials science become reality. In order to assess the quality of calculated material properties, Jacob’s ladder provides an informal classification, where exchange and correlation functionals of similar capabilities are placed at the same rung of this ladder, while improved and more accurate ones are placed at higher rungs. Climbing Jacob’s ladder, i.e. employing more accurate exchange and correlation functionals, increases the quality of the results and the computational demands, and provides some guidance as to what accuracies and computational costs to expect from specific calculations. This is particular important for materials whose electronic ground state properties are incorrectly described, e.g. small band gap semiconductors, and materials, where system sizes for subsequent investigations, like defect properties or band offsets in heterostructures, become prohibitively large for more accurate exchange and correlation functionals.Here, we provide a systematic density functional theory study on the ground state properties of Ag2ZnSnSe4 and Cu2ZnSnSe4 for the lowest four rungs of Jacob’s ladder. Cu2ZnSnSe4, and in particular its alloys with Ag, is a promising candidate material for future thin-film solar cell absorber layers. In the present work, the obtained material properties are compared to available experimental data, allowing to benchmark the accuracy of the employed exchange and correlation functionals. We also provide a comparative study for subsequent quasiparticle calculations. Therein, the influence of differently obtained eigenvalues and orbitals as starting points are critically assessed with respect to available experimental data. Our results show that, structural properties based on the SCAN functional show overall best agreement with available experimental data, whereas additional hybrid functional calculations are necessary for satisfying results on electronic and optical properties.

Novel ternary g-C3N4 nanosheet/Ag2MoO4/AgI photocatalysts: Impressive photocatalysts for removal of various contaminants

This study presents the combining g-C3N4 nanosheet (NSCN) with Ag2MoO4 and AgI semiconductors through facile precipitation method as ternary visible-light-driven photocatalysts. The photoability of the nanocomposites was evaluated via photodegradation of different aqueous contaminants under visible-light excitation. The results displayed that the contents of Ag2MoO4 and AgI has considerable impact on photoability of ternary nanocomposite and the NSCN/Ag2MoO4/AgI (30%) nanocomposite illustrated best activity in the removal of MB, RhB, fuchsine, and MO, which was about 109, 155, 55.9, and 51.8-times premier than the bare CN, respectively. In addition, the boosted ability of the ternary system is 27.4, 22.6, 22.9, and 23.7 times superior relative to the NSCN in removal of MB, RhB, fuchsine, and MO pollutants, respectively, whereas the ability is 7.2, 4.9, 3.7, and 3.4 times premier than that of the NSCN/Ag2MoO4 (20%) system. The improved photoability was assigned to the enhanced absorption of visible light owing to the existence of small band gap semiconductors and improvement of the segregation of the charges due to the construction of ternary nanocomposite. The reactive species scavenging results predicted that superoxide anion radicals are the essential involved species in the RhB elimination. In addition, by evaluating the electrochemical features, a suitable mechanism was offered based on the band energies to explain the increased charges segregation and movement, which led to admirable photoactivities in removals of several hazardous pollutants.

Synthesis and structural characterization of the new Zintl phases Ba3Cd2P4 and Ba2Cd2P3. Rare example of small gap semiconducting behavior with negative thermopower within the range 300 ​K–700 ​K

The new Zintl phases Ba3Cd2P4 and Ba2Cd2P3 have been synthesized using Pb flux, which allowed for the growth of 4–5 ​mm large crystals. The structures were determined utilizing single-crystal X-ray diffraction methods. Both compounds crystalize in the monoclinic crystal system (space group C2/m (No. 12)) and their structures are closely related. The structure of Ba3Cd2P4 can be seen as being comprised of divalent Ba atoms and conjoined CdP4 tetrahedra in the form of [Cd2P4]6– layers. Within the layers, homoatomic P–P bonds are present, which if cleaved, leave two infinite [CdP3]7– chains running along the crystallographic b-axis. The other structure, that of Ba2Cd2P3, can be rationalized as also having divalent Ba atoms and conjoined CdP4 tetrahedra in the form of [Cd2P3]6– layers. These layers, again, can be visualized as chains that run down the crystallographic b-axis, which are further connected by P–P dimers. Electronic band structure calculations show that each structure has an optimal number of valence electrons, and therefore conform to the Zintl-Klemm concept. Accordingly, the two compounds can be considered small band gap semiconductors, with band gaps of ca. 0.1 ​eV and 0.6 ​eV for Ba3Cd2P4 and Ba2Cd2P3, respectively. Electrical resistivity measurements show that Ba3Cd2P4 displays a large resistivity value at room temperature and an experimental band gap of ca. 0.05 ​eV, which fits reasonably well with the theoretical predictions. Thermopower measurements show that throughout the temperature range 300 ​K–700 ​K, Ba3Cd2P4 displays a negative Seebeck coefficient. The extremum value of −84 μV is reached at 630 ​K, suggestive of an n-type semiconductor, a rarity among Zintl phases.

Fabrication of Mg2Sn(111) film by molecular beam epitaxy

Magnesium stannide (Mg2Sn) is a small bandgap semiconductor of interest as a promising thermoelectric or optoelectronic material. Thin films of Mg2Sn were epitaxially grown on sapphire (0001) surfaces using molecular beam epitaxy. The epitaxial relationship is (111)Mg2Sn∥(0001)Al2O3 and [112¯]Mg2Sn∥[101¯0]Al2O3, with a small amount of stacking faults. A relatively high growth rate of 0.21–0.27 nm/s was attainable.

Comprehensive study of the physical properties of Ba3Pn2 (Pn=N, P, As, Sb and Bi) through first principles technique

The structural, electronic and optical properties of Ba3Pn2 (Pn=N, P, As, Sb and Bi) compounds have been investigated using density functional theory based on Full potential linearized augmented plane wave (FPLAPW) method, as implemented in WIEN2K package. Wu-Cohan approach is used for structural parameters; lattice constant, optimized volume, bulk modulus and its derivative. For band structure calculations Modified Beck and Johnson Approximation (mBJ) is used, which predicted these materials as small and direct band gap semiconductors. The bond lengths between Ba and Pn are also calculated. Ba-d state and Pn-p state contribute in VB as well as in CB. The bonding nature calculation indicates the strong covalent bond between Barium (Ba) and Pnictogen group situated at corner of the unit cell position and ionic bond between Ba and Pnictogen group placed at unit cell centre. The optical parameters, like dielectric function (real and imaginary part), optical conductivity, absorption coefficient, index of refraction (real and imaginary) and reflectivity are also calculated, which is the IR region of electromagnetic spectra.

BiOBr and AgBr co-modified ZnO photocatalyst: A novel nanocomposite with p-n-n heterojunctions for highly effective photocatalytic removal of organic contaminants

A series of ZnO/BiOBr/AgBr visible-light-induced photocatalysts were synthesized by a simple procedure. Among ternary nanocomposites, the ZnO/BiOBr/AgBr (30%) photocatalyst displayed the highest ability for photodegradation of MO, which was almost 95.3 and 3 times as high as the bare ZnO and binary ZnO/BiOBr (30%) nanocomposite, respectively. The promoted photocatalytic ability of the ZnO/BiOBr/AgBr (30%) photocatalyst was assigned to the improved absorption of visible light, rapid electron/hole separation, and boosted textural properties. It was proposed that construction of p-n-n heterojunctions between the counterparts could play pivotal role in separation of the charges. Furthermore, the O2−, OH, and h+ species were detected as the oxidative species in the photocatalytic system by the scavenging tests. The present work benefited the improvement of photocatalytic performance of ZnO through integration with two small band gap semiconductors for water purification.

(Invited) Photoelectrochemistry of Semiconducting Oxide Materials for Solar Water Splitting

There is a large family of binary, ternary, quaternary, etc. metal oxides that exhibit semiconducting properties, which makes them possible candidates to be applied in solar water splitting. With the recent advances in the controlled and economical synthesis of a large variety of metal oxides and their physical properties, new efforts in the photoelectrochemical characterization of such materials has become an important focus of research in the solar water splitting community.1,2 Although photoelectrochemical water splitting is an attractive method to convert solar energy to storable chemical energy in the form of hydrogen, the materials requirements to achieve this efficiently are very challenging. The semiconducting material needs to absorb sunlight efficiently, must to be capable of reducing and/or oxidizing water, and has to be stable under illumination under current flow in an aqueous electrolyte solution. In particular, for small bandgap semiconductors, stability is often an issue. In addition, the water reduction and oxidation reactions need to occur fast, in order to effectively compete with recombination or surface degradation processes. The kinetic rate constants for charge transfer and surface recombination therefore are important parameters, but these are difficult to obtain from steady-state measurements. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful option to study the carrier dynamics in a photoelectrochemical cell. The frequency-dependent photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge transfer and surface recombination can be determined using a simple model, depending on the complexity of the system under study.3 We have used IMPS on a variety of systems including WO3, p-CuBi2O4 and WO3-BiVO4 heterojunctions, in order to elucidate the rate determining steps in the charge carrier dynamics. For CuBi2O4 photocathodes, an unfavorable balance between the rate constants for charge transfer and surface recombination is found to limit the conversion efficiency.4 On the other hand, IMPS analysis of screen-printed WO3 photoanodes shows that the recombination rate constant is significantly smaller than the charge transfer rate constant; however, the film thickness plays an important role in collection efficiency.5 Recent results on WO3-BiVO4 heterojunctions, CuWO4, and electrocatalyzed Sn-doped Fe2O3 photoanodes are also discussed. References “Solar Hydrogen Generation: Toward a Renewable Energy Future”. K. Rajeshwar, R. McConnell and S. Licht, Editors, Kluwer Academic, New York (2008).“Solution combustion synthesis of oxide semiconductors for solar energy conversion and environmental remediation”. K. Rajeshwar and N. R. de Tacconi, Chem. Soc. Rev. 38(7) 1984-1998 (2009).“Photoelectrochemical Water Splitting at Semiconductor Electrodes: Fundamental Problems and New Perspectives”. L. M. Peter and K. G. Upul Wijayantha, ChemPhysChem, 15, 1983–1995 (2014).“Charge Transfer and Recombination Dynamics at Inkjet-Printed CuBi2O4 Electrodes for Photoelectrochemical Water Splitting”. Ingrid Rodríguez-Gutiérrez, Rodrigo García-Rodríguez, Manuel Rodríguez-Pérez, AlbertoVega-Poot, Geonel Rodríguez Gattorno, Bruce A. Parkinson, and Gerko Oskam. J. Phys. Chem. C, 122, 27169−27179 (2018).“Charge Transfer and Recombination Kinetics at WO3 for Photoelectrochemical Water Oxidation”. Manuel Rodríguez-Pérez, Ingrid Rodríguez-Gutiérrez, Alberto Vega-Poot, Rodrigo García-Rodríguez, Geonel Rodríguez-Gattorno, Gerko Oskam. Electrochim. Acta, 258, 900-908 (2017).

Pancakes under Pressure: A Case Study on Isostructural Dithia- and Diselenadiazolyl Radical Dimers.

The isostructural dimers of the 1,4-phenylene-bridged bis-1,2,3,5-dithia- and bis-1,2,3,5-diselenadiazolyl diradicals 1,4-S/Se are small band gap semiconductors. The response of their molecular and solid state electronic structures to pressure has been explored over the range 0-10 GPa. The crystal structures, which consist of cofacially aligned (pancake) π-dimers packed into herringbone arrays, experience a continuous, near-isotropic compression. While the intramolecular covalent E-E (E = S/Se) bonds remain relatively unchanged with pressurization, the intradimer E···E separations are significantly shortened. Molecular and band electronic structure calculations using density functional theory methods indicate that compression of the π-dimers leads to a widening of the gap Δ E between the highest occupied and lowest unoccupied molecular orbitals of the dimer, an effect that offsets the expected decrease in the valence-to-conduction band gap Eg occasioned by pressure-induced spreading of the valence and conduction bands. Consistent with the predicted consequences of this competition between intra- and interdimer interactions, variable temperature high pressure conductivity measurements reveal at best an order-of-magnitude increase in conductivity with pressure for the two compounds over the pressure range 0-10 GPa. While a small reduction in the thermal activation energy Eact with increasing pressure is observed, extrapolation of the rate of decrease suggests a projected onset of metallization ( Eact ≈ 0) in excess of 20 GPa.

Density functional theory study on the effect of tensile deformation on the electrical structure of O adsorbed graphyne

The effects of tensile deformation on the stability and electronic structure of the O adsorbed graphyne system is studied by using density functional theory based on the first-principles. It is found that in the current research range, the tensile deformation causes the structural stability of the intrinsic graphyne to decrease slightly, but the stability of the O adsorbed graphyne system increases. Compared with the undrawn stretching system, the adsorption energy of the O adsorbed graphyne system which is subjected to stretching increases. The adsorption energy is high in small deformation. Then the adsorption energy becomes smaller as the tensile deformation increases. It is proved that the tensile deformation can exacerbate the adsorption of O atoms. After the intrinsic graphyne is subjected to stretching, the band gap decreases as the tensile deformation increases. The graphyne is changed from a medium band gap semiconductor to a small band gap semiconductor. After the graphyne adsorbs the O atom, the C atom which is closest to the O atom is pulled up, the plane of the graphyne is twisted, the band gap of the graphyne is reduced, and a new impurity level is introduced at the Fermi level. The adsorption of graphyne on O atoms belongs to the category of chemical adsorption, and C atoms have stable bonding. There is a strong electron transfer between them, causing the Fermi level to move. Compared with the density of states of the graphyne, multiple density peaks appear in the density of states. A very pronounced energy gap has also appeared near the energy level of −18 eV. After applying uniaxial tensile deformation, the band gap is reduced from 0.263 eV to 0 eV, exhibiting the Dirac cone, corresponding to the metalloid properties.

Interfacial engineering of Fe2O3@BOC heterojunction for efficient detoxification of toxic metal and dye under visible light illumination

Recent developments of small band gap semiconductor coupled bismuth carbonate (BOC) heterojunction are advantageous for photocatalysis application because of their improved solar harvesting ability and enhanced charge-carrier collection. In this work, we have developed iron (III) oxide decorated bismuth carbonate (Fe2O3@BOC) heterojunction via a simple two-step process. The developed heterojunction exhibits excellent photocatalytic activity towards reduction of carcinogenic and mutagenic Cr(VI) to nontoxic Cr(III) and degradation of toxic dye [methylene blue (MB)] under visible light illumination. Further investigation revealed that the loading of Fe2O3 nanoparticles had an impact on efficient charge carrier collection at the interface of Fe2O3@BOC heterojunctions. The unprecedented photocatalytic activity for Fe2O3@BOC1 heterojunction at room temperature could be attributed due to the enhancement in light absorption ability and suppression of electron–hole pair recombination at the heterojunction interface. In addition, reduction in efficacy of the heterojunction with increase in loading of Fe2O3 nanoparticles on BOC surface further confirms the role of interface on the modulation of photocatalytic activity. The role of photogenerated electrons and reactive oxygen species involved during photocatalytic reduction of Cr(VI) and degradation of MB was studied in detail. Moreover, recyclability experiment demonstrates that the developed photocatalyst can be reused without decay in performance. Finally, development of inexpensive prototype reactor is demonstrated towards reduction of Cr(VI) and degradation of MB under continuous flow operation. Thus, good efficacy of the developed reactor for cleaning of toxic pollutants in water makes the heterojunction (Fe2O3@BOC1) a promising photocatalyst for water purification in near future.

Photo-excited dynamics in the excitonic insulator Ta2NiSe5

The excitonic insulator is an intriguing correlated electron phase formed of condensed excitons. A promising candidate is the small band gap semiconductor Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally developed by Kabanov et al for superconductors (Kabanov et al 1999 Phys. Rev. B 59 1497), we show that the material’s intrinsic gap can be described as almost temperature independent for temperatures up to about 250 K to 275 K. This behavior supports the existence of the excitonic insulator state in Ta2NiSe5. The onset of an additional temperature dependent component to the gap above these temperatures suggests that the material is located in the BEC-BCS crossover regime. Furthermore, we show that this state is very stable against strong photoexcitation, which reveals that the free charge carriers are unable to effectively screen the attractive Coulomb interaction between electrons and holes, likely due to the quasi 1D structure of Ta2NiSe5.

Light metal decorated graphdiyne nanosheets for reversible hydrogen storage

The sensitive nature of molecular hydrogen (H2) interaction with the surfaces of pristine and functionalized nanostructures, especially two-dimensional materials, has been a subject of debate for a while now. An accurate approximation of the H2 adsorption mechanism has vital significance for fields such as H2 storage applications. Owing to the importance of this issue, we have performed a comprehensive density functional theory (DFT) study by means of several different approximations to investigate the structural, electronic, charge transfer and energy storage properties of pristine and functionalized graphdiyne (GDY) nanosheets. The dopants considered here include the light metals Li, Na, K, Ca, Sc and Ti, which have a uniform distribution over GDY even at high doping concentration due to their strong binding and charge transfer mechanism. Upon 11% of metal functionalization, GDY changes into a metallic state from being a small band-gap semiconductor. Such situations turn the dopants to a partial positive state, which is favorable for adsorption of H2 molecules. The adsorption mechanism of H2 on GDY has been studied and compared by different methods like generalized gradient approximation, van der Waals density functional and DFT-D3 functionals. It has been established that each functionalized system anchors multiple H2 molecules with adsorption energies that fall into a suitable range regardless of the functional used for approximations. A significantly high H2 storage capacity would guarantee that light metal-doped GDY nanosheets could serve as efficient and reversible H2 storage materials.

Strong Coupling Nature of the Excitonic Insulator State in Ta_{2}NiSe_{5}.

We analyze the measured optical conductivity spectra using the density-functional-theory-based electronic structure calculation and density-matrix renormalization group calculation of an effective model. We show that, in contrast to a conventional description, the Bose-Einstein condensation of preformed excitons occurs in Ta_{2}NiSe_{5}, despite the fact that a noninteracting band structure is a band-overlap semimetal rather than a small band-gap semiconductor. The system above the transition temperature is therefore not a semimetal but rather a state of preformed excitons with a finite band gap. A novel insulator state caused by the strong electron-hole attraction is thus established in a real material.

Density functional theory study on the electronic structure and optical properties of S absorbed graphene

The effect of bend angle on the stability, electronic structure and optical properties of S absorbed graphene system is studied by using density functional theory based on the first - principles. Within the scope of the current study, it is found that the C atoms which are closest to the S atom are pulled up, the graphene plane is distorted, and the band gap of the graphene is opened, which changes graphene from quasi-metal to semiconductors. Bending deformation decreases the structural stability of S atom adsorbed on graphene system. The adsorption energy decreases with the increase of bending angle. After the intrinsic graphene is bent, the band gap is opened, and the bandgap increases with the bending angle. When the bending angle is between 0°–20°, it still corresponds to the quasi-metal property. When the bending angle increases to 25°, it gradually changes from small bandgap semiconductor to medium bandgap semiconductor. The band gap of graphene adsorbed S atom system decreases with the increase of the bending angle, and it always corresponds to medium bandgap semiconductors. When the angle is increased to 30°, the system loses its stability. The rate of increase of absorption coefficient increases with the increase of bending angle. The deformation causes the maximum absorption peak and the maximum reflection peak of the system to red shift, and the degree of red shift increases with the increase of the bending angle.

Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for Molecular Semiconductors and Metals.

In pursuit of closed-shell building blocks for single-component organic semiconductors and metals, we have prepared benzoquino-bis-1,2,3-thiaselenazole QS, a heterocyclic selenium-based zwitterion with a small gap (λmax = 729 nm) between its highest occupied and lowest unoccupied molecular orbitals. In the solid state, QS exists in two crystalline phases and one nanocrystalline phase. The structures of the crystalline phases (space groups R3 c and P21/ c) have been determined by high-resolution powder X-ray diffraction methods at ambient and elevated pressures (0-15 GPa), and their crystal packing patterns have been compared with that of the related all-sulfur zwitterion benzoquino-bis-1,2,3-dithiazole QT (space group Cmc21). Structural differences between the S- and Se-based materials are interpreted in terms of local intermolecular S/Se···N'/O' secondary bonding interactions, the strength of which varies with the nature of the chalcogen (S vs Se). While the perfectly two-dimensional "brick-wall" packing pattern associated with the Cmc21 phase of QT is not found for QS, all three phases of QS are nonetheless small band gap semiconductors, with σRT ranging from 10-5 S cm-1 for the P21/ c phase to 10-3 S cm-1 for the R3 c phase. The bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy Eact. For the R3 c phase, band gap closure to yield an organic molecular metal with a σRT of ∼102 S cm-1 occurs at 6 GPa. Band gaps estimated from density functional theory band structure calculations on the ambient- and high-pressure crystal structures of QT and QS correlate well with those obtained experimentally.

Temperature-Dependent and Gate-Tunable Rectification in a Black Phosphorus/WS2 van der Waals Heterojunction Diode.

Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention because of their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p-n diode functionality. In this study, for the first time, we fabricated a heterojunction p-n diode composed of BP and WS2. The rectification effects are examined for monolayer, bilayer, trilayer, and multilayer WS2 flakes in our BP/WS2 van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6 × 104, temperature dependence, thickness dependence of rectification, and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS2 van der Waals p-n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS2 van der Waals p-n diodes have a profound potential to fabricate rectifiers, solar cells, and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

Near-field three-terminal thermoelectric heat engine

We propose a near-field inelastic thermoelectric heat engine where quantum-dots are used to effectively rectify the charge flow of photo-carriers. The device converts near-field heat radiation into useful electrical power. Heat absorption and inelastic transport can be enhanced by introducing two continuous spectra separated by an energy gap. The thermoelectric transport properties of the heat engine are studied in the linear-response regime. Using a small band-gap semiconductor as the absorption material, we show that the device achieves very large thermopower and thermoelectric figure-of-merit, as well as considerable power-factor. By analyzing thermal-photo-carrier generation and conduction, we reveal that the Seebeck coefficient and the figure of merit have oscillatory dependence on the thickness of the vacuum gap. Meanwhile, the power-factor, the charge and thermal conductivity are significantly improved by near-field radiation. Conditions and guiding principles for powerful and efficient thermoelectric heat engines are discussed in details.

CsMn4As3: A Layered Tetragonal Transition-Metal Pnictide Compound with an Antiferromagnetic Ground State.

We report the synthesis and properties of a new layered tetragonal ternary compound CsMn4As3 (structure type, KCu4S3; space group, P4/ mmm, no. 123; and Z = 2). The material is a small band gap semiconductor and exhibits an antiferromagnetic ground state associated with Mn spins. The compound exhibits a signature of a distinct magnetic moment canting event at 150(5) K with a canting angle ≈ 0.3°. Although some features of the magnetic characteristics of this new compound are qualitatively similar to those of the related BaMn2As2, the underlying Mn sublattices of the two materials are quite different. While the Mn square-lattice layers in BaMn2As2 are equally spaced along the c-direction with the interlayer distance dLBa = 6.7341(4) Å, the Mn sublattice forms bilayers in CsMn4As3 with the interlayer distance within a bilayer being dLCs = 3.1661(6) Å; the distance between the two adjacent bilayers is dB = 7.290(6) Å. This difference in the Mn sublattice is bound to significantly alter the energy balance among the J1, J2, and J c exchange interactions within the J1- J2- J c model compared to those in BaMn2As2 and the other related 122 compounds, including the well-known iron-arsenide superconductor parent compound BaFe2As2. Owing to the novelty of its transition-metal sublattice, this new addition to the family of tetragonal materials related to the iron-based superconductors brings prospects for doping and pressure studies in the search of new superconducting phases as well as other exciting correlated electron properties.