Narrow Gap Materials Research Articles

Electron–phonon coupling and a resonant-like optical observation of a band inversion in topological crystal insulator Pb1−x Sn x Se

The optical reflectivity of n-type Pb0.865Sn0.135Se and Pb0.75Sn0.25Se solid solutions was measured in the THz spectral region energetically corresponding to optical phonon excitations and in the temperature range from 40 K to 280 K. The first solid solution exhibits an open energy gap with trivial band ordering at all temperatures, while for the second one the transition from trivial insulator to topological crystal insulator phase is expected. The analysis of Pb0.75Sn0.25Se data performed within the dynamic dielectric function formalism revealed an anomaly of resonance-like character in the temperature dependence of LO phonon frequency. The formula for LO phonon energy renormalization reproduced observed anomaly for energy gap equal to zero, the phase transition occurred at T 0 = (172 ± 2) K. This effect was absent for Pb0.865Sn0.135Se. Present results show that reflectivity measurements in the THz range in the vicinity of LO phonon frequency can be valuable experimental method for precise determining of band inversion temperature in narrow-gap topological materials.

Absorption in narrow and wide gap materials

The absorption spectrum of a material reveals the absorbed light frequencies, characteristic peaks, and the line width of absorption bands. This information is critical for understanding the energy levels involved in the absorption process as well as the material's electronic structure. In this study, an equation connecting the absorption line width with the static dielectric function is derived for narrow and wide gap materials. It is then compared with the Penn model. It has been found that the constant in the Penn model has a value that is restricted to the range of 0.5 to 1. Application of this equation to various narrow and wide gap materials is then discussed.

Near-Infrared Light Driven ZnIn2S4-Based Photocatalysts for Environmental and Energy Applications: Progress and Perspectives.

Zinc indium sulfide (ZnIn2S4), as a significant visible-light-responsive photocatalyst, has become a research hotspot to tackle energy demand and environmental issues owing to its excellent properties of high stability, easy fabrication, and remarkable catalytic activity. However, its drawbacks, including low utilization of solar light and fast photoinduced charge carriers, limit its applications. Promoting the response for near-infrared (NIR) light (~52% solar light) of ZnIn2S4-based photocatalysts is the primary challenge to overcome. In this review, various modulation strategies of ZnIn2S4 have been described, which include hybrid with narrow optical gap materials, bandgap engineering, up-conversion materials, and surface plasmon materials for enhanced NIR photocatalytic performance in the applications of hydrogen evolution, pollutants purification, and CO2 reduction. In addition, the synthesis methods and mechanisms of NIR light-driven ZnIn2S4-based photocatalysts are summarized. Finally, this review presents perspectives for future development of efficient NIR photon conversion of ZnIn2S4-based photocatalysts.

Optimized molecular beam epitaxy process for lattice-matched narrow-bandgap (0.8 eV) GaInNAsSb solar junctions

High performance narrow-bandgap GaInNAsSb solar cells are instrumental for the development of lattice-matched GaAs-based solar cells with more than four junctions. To this end a comprehensive optimization process including the effects of growth temperature, As/III beam equivalent pressure ratio, and Sb flux on the performance of 0.8 eV GaInNAsSb solar cells grown by molecular beam epitaxy is reported. For this, sets of GaInNAsSb p-i-n solar cell structures with 5–6% nitrogen compositions were fabricated, while varying the key growth parameters. The quantum efficiency and current generation increased significantly when the narrow gap materials were grown at elevated growth temperatures, close to phase separation. A further improvement in the current generation was observed by employing lower As/III beam equivalent pressure ratios. The best GaInNAsSb cell exhibited about 94% peak external quantum efficiency and generated a short-circuit current of 17.7 mA/cm 2 with AM1.5D (1000 W/m 2 ) illumination at wavelengths above 900 nm without employing a back surface reflector. Our analysis indicates that the best cell is already close to being absorption limited. While the N composition should be kept as low as possible (i.e., ≲5%) to achieve high performance, increasing the Sb flux generally results in improved the material quality, i.e., leading to a slight improvement for the open-circuit voltages and fill factors. In addition, it was found that the phase separation observed at the growth temperature of 480 °C could effectively be inhibited by employing higher Sb fluxes. • Optimized MBE growth for 0.8 eV GaInNAsSb solar junctions with 5–6% N content. • High current obtained using high growth temperatures and lower As pressures. • High peak external quantum efficiency of 94% demonstrated. • Background doping level as low as 1.1 × 10 15 cm −3 for GaInNAsSb with 6% N. • Phase separation is suppressed, and power output improved by optimizing Sb flux.

Optical transitions, exciton radiative decay, and valley coherence in lead chalcogenide quantum dots

We propose the concept of valley coherence and superradiance in the reciprocal space and show that it leads to an $N$-fold decrease of the bright exciton radiative lifetime in quantum dots (QDs) of an $N$-valley semiconductor. Next we explain why, despite this, the exciton radiative lifetimes in PbX (X = S, Se, Te) QDs, measured from the photoluminescence decay, are in the microsecond range. We also address peculiarities of the light-matter interaction in nanostructures made of narrow-gap materials with strong inter-band coupling.

Optimizing topological switching in confined 2D-Xene nanoribbons via finite-size effects

In a blueprint for topological electronics, edge state transport in a topological insulator material can be controlled by employing a gate-induced topological quantum phase transition. Here, by studying the width dependence of electronic properties, it is inferred that zigzag-Xene nanoribbons are promising materials for topological electronics with a display of unique physical characteristics associated with the intrinsic band topology and the finite-size effects on gate-induced topological switching. First, due to intertwining with intrinsic band topology-driven energy-zero modes in the pristine case, spin-filtered chiral edge states in zigzag-Xene nanoribbons remain gapless and protected against backward scattering even with finite inter-edge overlapping in ultra-narrow ribbons, i.e., a 2D quantum spin Hall material turns into a 1D topological metal. Second, mainly due to width- and momentum-dependent tunability of the gate-induced inter-edge coupling, the threshold-voltage required for switching between gapless and gapped edge states reduces as the width decreases, without any fundamental lower bound. Third, when the width of zigzag-Xene nanoribbons is smaller than a critical limit, topological switching between edge states can be attained without bulk bandgap closing and reopening. This is primarily due to the quantum confinement effect on the bulk band spectrum, which increases the nontrivial bulk bandgap with decrease in width. The existence of such protected gapless edge states and reduction in threshold-voltage accompanied by enhancement in the bulk bandgap overturns the general wisdom of utilizing narrow-gap and wide channel materials for reducing the threshold-voltage in a standard field effect transistor analysis and paves the way toward low-voltage topological devices.

Performance comparison of hybrid functionals for describing narrow-gap semiconductors: A study on low-temperature thermoelectric material α-SrSi2

Narrow-bandgap thermoelectric semiconductors with precisely designed electronic structures are required for highly efficient energy-recycling societies. As a solution to the underestimation of the bandgap value, which is the most serious problem of first-principles calculations based on the density functional theory (DFT) especially for narrow-gap materials, we consider the PBE0 functional, Heyd–Scuseria–Ernzerhof screening hybrid functional (HSE06), and recently developed Gaussian–Perdew–Burke–Ernzerhof (Gau–PBE) hybrid functional. The convergence of the energy and bandgap of extremely narrow-gap semiconductor α-SrSi2 with respect to the mesh parameters, which are the number of k-point divisions in DFT and Hartree–Fock (HF) exchange integral calculations, was investigated using each functional, and the electronic structure and transport properties were then calculated. Consequently, Gau–PBE provided similar electronic and transport properties to those obtained using the other methods while lowering the calculation costs, i.e., with >80% less computation time and a smaller number of HF mesh points. These results provide insights to the low-cost practical calculation of the precise transport properties of low-temperature thermoelectric materials.

Magneto-transport evidence for strong topological insulator phase in ZrTe5

The identification of a non-trivial band topology usually relies on directly probing the protected surface/edge states. But, it is difficult to achieve electronically in narrow-gap topological materials due to the small (meV) energy scales. Here, we demonstrate that band inversion, a crucial ingredient of the non-trivial band topology, can serve as an alternative, experimentally accessible indicator. We show that an inverted band can lead to a four-fold splitting of the non-zero Landau levels, contrasting the two-fold splitting (spin splitting only) in the normal band. We confirm our predictions in magneto-transport experiments on a narrow-gap strong topological insulator, zirconium pentatelluride (ZrTe5), with the observation of additional splittings in the quantum oscillations and also an anomalous peak in the extreme quantum limit. Our work establishes an effective strategy for identifying the band inversion as well as the associated topological phases for future topological materials research.

Three-dimensional luminescence microscopy for quantitative plasma characterization in bulk semiconductors

Important challenges remain in the development of ultrafast laser writing inside semiconductor materials because the properties of narrow gap materials cause strong propagation distortions to intense infrared light. Here, we introduce a simple and robust imaging method for high-dynamic-range investigations of the laser–matter interactions in bulk semiconductors. Supported by measurements in gallium arsenide and silicon, we show how z-scan imaging of the band-to-band radiative recombination enables quantitative reconstruction of the three-dimensional distributions of free-carriers generated by nonlinear ionization with ultrashort pulses. The validity is confirmed by comparisons with ultrafast transmission microscopy (shadowgraphy) images. The superior sensitivity of the zero-background luminescence method allows the measurement of local carrier densities as low as ≈1016 cm−3 inside GaAs that is inaccessible by shadowgraphy. It provides the first direct evidence of the low density plasma generated far prior to the focus that causes the previously reported intensity clamping phenomenon. The potential of this non-coherent 3D imaging method to assess complex beam distortion features is also exemplified by real-time pre-compensation of aberration for an intense interacting beam.

Strategies to extend near-infrared light harvest of polymer carbon nitride photocatalysts

Graphitic carbon nitride (g-C3N4), as a significant metal-free photocatalyst, has elicited ripples of excitement due to its many extraordinary features, such as its mild bandgap, high thermal and chemical stability, inexpansive, and environmental friendly. However, the unsatisfactory solar light absorption, low surface area and the rapid recombination of photogenerated charges severely restrict the photocatalytic activity of bulk g-C3N4. Especially, g-C3N4 with a bandgap of 2.7 eV means an absorption threshold at about 450 nm that is still insufficient for efficient visible light capturing and leaves the near-infrared (NIR) light spectrum unexploited. In order to facilitate future material design for efficient g-C3N4 photocatalysts under solar light (UV ~5%, visible light ~43%, NIR light ~52%), we reviewed the recent progress of NIR-driven g-C3N4 based photocatalysts. Many strategies, including combination of narrow optical gap materials, bandgap engineering, upconversion materials, plasmonic materials, and photosensitizers, have been summarized to broaden the light absorption of g-C3N4 to NIR light region. Besides, the diverse photocatalytic applications of NIR driven g-C3N4 photocatalysts have been summarized, including water purification, water splitting, N2 photofixation, CO2 reduction, NO removal, H2O2 production, bacteria disinfection, photodynamic therapy and organic synthesis, etc. The mechanism and the roles of various strategies in the NIR photocatalytic process were highlighted in details. Moreover, the limitations and possible solutions for each method are discussed.

Prediction of tunable spin-orbit gapped materials for dark matter detection

New ideas for low-mass dark matter direct detection suggest that narrow band gap materials, such as Dirac semiconductors, are sensitive to the absorption of meV dark matter or the scattering of keV dark matter. Here we propose spin-orbit semiconductors - materials whose band gap arises due to spin-orbit coupling - as low-mass dark matter targets owing to their ~10 meV band gaps. We present three material families that are predicted to be spin-orbit semiconductors using Density Functional Theory (DFT), assess their electronic and topological features, and evaluate their use as low-mass dark matter targets. In particular, we find that that the tin pnictide compounds are especially suitable having a tunable range of meV-scale band gaps with anisotropic Fermi velocities allowing directional detection. Finally, we address the pitfalls in the DFT methods that must be considered in the ab initio prediction of narrow-gapped materials, including those close to the topological critical point.

Correction to “Ultra-High Thermoelectric Power Factors in Narrow Gap Materials with Asymmetric Bands”

Correction to “Ultra-High Thermoelectric Power Factors in Narrow Gap Materials with Asymmetric Bands”

Unraveling electronic band structure of narrow-bandgap p-n nanojunctions in heterostructured nanowires.

The electronic band structure of complex nanostructured semiconductors has a considerable effect on the final electronic and optical properties of the material and, ultimately, on the functionality of the devices incorporating them. Valence electron energy-loss spectroscopy (VEELS) in the transmission electron microscope (TEM) provides the possibility of measuring this property of semiconductors with high spatial resolution. However, it still represents a challenge for narrow-bandgap semiconductors, since an electron beam with low energy spread is required. Here we demonstrate that by means of monochromated VEELS we can study the electronic band structure of narrow-gap materials GaSb and InAs in the form of heterostructured nanowires, with bandgap values down to 0.5 eV, especially important for newly developed structures with unknown bandgaps. Using complex heterostructured InAs-GaSb nanowires, we determine a bandgap value of 0.54 eV for wurtzite InAs. Moreover, we directly compare the bandgaps of wurtzite and zinc blende polytypes of GaSb in a single nanostructure, measured here as 0.84 and 0.75 eV, respectively. This allows us to solve an existing controversy in the band alignment between these structures arising from theoretical predictions. The findings demonstrate the potential of monochromated VEELS to provide a better understanding of the band alignment at the heterointerfaces of narrow-bandgap complex nanostructured materials with high spatial resolution. This is especially important for semiconductor device applications where even the slightest variations of the electronic band structure at the nanoscale can play a crucial role in their functionality.

The effective mass, vibration and electromagnetic properties of tetragonal iron aluminide FeAl2

MoSi2-type tetragonal iron dialuminide (space group I4/mmm) have been successfully synthesized and determined in a previous stud by Tobita et al. In this work, the electronic structure, effective mass, vibration, and electromagnetic properties of t-FeAl2 were predicted using the density functional theory (DFT). First, the predicted crystal lattice parameters reproduced the previously reported experimental data. Second, the electronic structure shows that t-FeAl2 is a narrow-gap semiconductor material with an indirect gap (0.059 eV), and the effective masse of the electron (me*) and hole (mh*) is 0.35 m0 and 0.2 m0, respectively. Third, the electromagnetic wave absorption characteristics such as the frequency-dependence of the relative complex permittivity, dielectric loss tangent tgδ, electrical conductivity, quality factor Q × f and reflection loss are calculated. Besides, using the linear response theory, the phonon dispersion curves and vibrational properties are calculated. The phonon dispersion curves do not have imaginary phonon frequencies, confirming the dynamic stability of t-FeAl2. Finally, the vibrational properties are investigated, and the infrared (IR)-active and Raman-active modes are also assigned and briefly discussed.

Pulse-duration dependence of laser-induced modifications inside silicon.

The advent of ultrafast infrared lasers provides a unique opportunity for direct fabrication of three-dimensional silicon microdevices. However, strong nonlinearities prevent access to modification regimes in narrow gap materials with the shortest laser pulses. In contrary to surface experiments for which one can always define an energy threshold to initiate modifications, we establish that some other threshold conditions inevitably apply on the pulse duration and the numerical aperture for focusing. In an experiment where we can vary continuously the pulse duration from 4 to 21 ps, we show that a minimum duration of 5.4 ps and a focusing numerical aperture of 0.85 are required to successfully initiate modifications. Below and above thresholds, we investigate the pulse duration dependence of the conditions applied in matter. Despite a modest pulse duration dependence of the energy threshold in the tested range, we found that all pulse durations are not equally performing to achieve highly reproducible modifications. Taken together with previous reports in the femtosecond and nanosecond regimes, this provides important guidelines on the appropriate conditions for internal structuring of silicon.

Ultra-High Thermoelectric Power Factors in Narrow Gap Materials with Asymmetric Bands

We theoretically unveil the unconventional possibility to achieve extremely high thermoelectric power factors in lightly doped narrow gap semiconductors with asymmetric conduction/valence bands operated in the bipolar transport regime. Specifically, using Boltzmann transport simulations, we show that narrow bandgap materials, rather than suffering from performance degradation due to bipolar conduction, if they possess highly asymmetric conduction and valence bands in terms of either effective masses, density of states, or phonon scattering rates, then they can deliver very high power factors. We show that this is reached because, under these conditions, electronic transport becomes phonon scattering limited, rather than ionized impurity scattering limited, which allows large conductivities. We explain why this effect has not been observed so far in the known narrow-gap semiconductors, interpret some recent related experimental findings, and propose a few examples from the half-Heusler materials family for which this effect can be observed and power factors even up to 50 mW/m$K^2$ can be reached.

Band edge thermometry for the MBE growth of (Hg,Cd)Te-based materials

The material system (Hg,Cd)Te is a prototype system for topological insulators and Dirac semi-metals. Molecular beam epitaxy is the preferred method for the growth of (Hg,Cd)Te epilayer heterostructures and CdTe-HgTe core-shell nanowires. Unfortunately, the high crystalline quality desired for charge transport investigations is only achieved for a very narrow window of the growth temperature. Additionally, the low growth temperature requires a special thermometry. Here, we demonstrate the improvement of the growth of (Hg,Cd)Te epilayer heterostructures and CdTe nanowires by the use of band edge thermometry. We show that even for narrow-gap materials band edge thermometry provides a suitable method to control the temperature directly at the growth front with a high precision and reproducibility, in contrast to a conventionally used thermocouple.

Spherical topological insulator nanoparticles: Quantum size effects and optical transitions

We have investigated the interplay between band inversion and size quantization in spherically shaped nanoparticles made from topological-insulator (TI) materials. A general theoretical framework is developed based on a versatile continuum-model description of the TI bulk band structure and the assumption of a hard-wall mass confinement. Analytical results are obtained for the wave functions of single-electron energy eigenstates and the matrix elements for optical transitions between them. As expected from spherical symmetry, quantized levels in TI nanoparticles can be labeled by quantum numbers $j$ and $m=-j, -j+1, \dots, j$ for total angular momentum and its projection on an arbitrary axis. The fact that TIs are narrow-gap materials, where the charge-carrier dynamics is described by a type of two-flavor Dirac model, requires $j$ to assume half-integer values and also causes a doubling of energy-level degeneracy where two different classes of states are distinguished by being parity eigenstates with eigenvalues $(-1)^{j\mp 1/2}$. The existence of energy eigenstates having the same $j$ but opposite parity enables optical transitions where $j$ is conserved, in addition to those adhering to the familiar selection rule where $j$ changes by $\pm 1$. All optical transitions satisfy the usual selection rule $\Delta m = 0, \pm 1$. We treat intra- and inter-band optical transitions on the same footing and establish ways for observing unusual quantum-size effects in TI nanoparticles, including oscillatory dependences of the band gap and of transition amplitudes on the nanoparticle radius. Our theory also provides a unified perspective on multi-band models for charge carriers in semiconductors and Dirac fermions from elementary-particle physics.

Approaching Quantization in Macroscopic Quantum Spin Hall Devices through Gate Training.

Quantum spin Hall edge channels hold great promise as dissipationless one-dimensional conductors. However, the ideal quantized conductance of 2e^{2}/h is only found in very short channels-in contradiction with the expected protection against backscattering of the topological insulator state. In this Letter we show that enhancing the band gap does not improve quantization. When we instead alter the potential landscape by charging trap states in the gate dielectric using gate training, we approach conductance quantization for macroscopically long channels. Effectively, the scattering length increases to 175 μm, more than 1 order of magnitude longer than in previous works for HgTe-based quantum wells. Our experiments show that the distortion of the potential landscape by impurities, leading to puddle formation in the narrow gap material, is the major obstacle for observing undisturbed quantum spin Hall edge channel transport.

Visible-to-NIR Photon Harvesting: Progressive Engineering of Catalysts for Solar-Powered Environmental Purification and Fuel Production.

Utilization of diffusive solar energy through photocatalytic processes for environmental purification and fuel production has long been pursued. However, efficient capture of visible-near-infrared (NIR) photons, especially for those with wavelengths longer than 600 nm, is a demanding quest in photocatalysis owing to their relatively low energy. In recent years, benefiting from the advances in photoactive material design, photocatalytic reaction system optimization, and new emerging mechanisms for long-wavelength photon activation, increasing numbers of studies on the harnessing of visible-NIR light for solar-to-chemical energy conversion have been reported. Here, the aim is to comprehensively summarize the progress in this area. The main strategies of the long-wavelength visible-NIR photon capture and the explicitly engineered material systems, i.e., narrow optical gap, photosensitizers, upconversion, and photothermal materials, are elaborated. In addition, the advances in long-wavelength light-driven photo- and photothermal-catalytic environmental remediation and fuel production are discussed. It is anticipated that this review presents the forefront achievements in visible-NIR photon capture and at the same time promotes the development of novel visible-NIR photon harnessing catalysts toward efficient solar energy utilization.