Showing papers on "Direct and indirect band gaps published in 2016"
TL;DR: The double perovskites Cs2AgBiBr6 and Cs 2AgBiCl6 have been synthesized from both solid state and solution routes, and X-ray diffraction measurements reveal band gaps of 2.19 eV and 2.77 eV as discussed by the authors.
Abstract: The double perovskites Cs2AgBiBr6 and Cs2AgBiCl6 have been synthesized from both solid state and solution routes. X-ray diffraction measurements show that both compounds adopt the cubic double perovskite structure, space group Fm3m, with lattice parameters of 11.2711(1) A (X = Br) and 10.7774(2) A (X = Cl). Diffuse reflectance measurements reveal band gaps of 2.19 eV (X = Br) and 2.77 eV (X = Cl) that are slightly smaller than the band gaps of the analogous lead halide perovskites, 2.26 eV for CH3NH3PbBr3 and 3.00 eV for CH3NH3PbCl3. Band structure calculations indicate that the interaction between the Ag 4d-orbitals and the 3p/4p-orbitals of the halide ion modifies the valence band leading to an indirect band gap. Both compounds are stable when exposed to air, but Cs2AgBiBr6 degrades over a period of weeks when exposed to both ambient air and light. These results show that halide double perovskite semiconductors are potentially an environmentally friendly alternative to the lead halide perovskite semico...
958 citations
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TL;DR: In this article, the authors proposed a new class of halide double perovskites, where the B$^{3+}$ and B$€ 2+} cations are In$^{2+} and Ag$^{+}, respectively.
Abstract: A$_2$BB$^\prime$X$_6$ halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally-friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs$_2$BiAgX$_6$ (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B$^{3+}$ and B$^{+}$ cations are In$^{3+}$ and Ag$^{+}$, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs$_2$InAgX$_6$ (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs$_2$InAgCl$_6$ and Cs$_2$InAgBr$_6$, and we succeed to form the hitherto unknown double perovskite Cs$_2$InAgCl$_6$. X-ray diffraction yields a double perovskite structure with space group $Fm\overline{3}m$. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs$_2$InAgX$_6$ and their mixed halides based on Goldschmidt's rules, and we find that it should also be possible to form Cs$_2$InAg(Cl$_{1-x}$Br$_{x}$)$_6$ for $x<1$. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.
519 citations
TL;DR: Valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites are reported, revealing an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS in these compounds.
Abstract: We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI3, MAPbBr3, CsPbBr3), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI3 VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic sta...
324 citations
TL;DR: It is shown that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene, however, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed.
Abstract: Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show th...
293 citations
TL;DR: In this article, a new method was proposed for extracting a direct optical band gap from absorption spectra of degenerately-doped bulk semiconductors, which was applied to pseudo-absorption spectra converted from diffuse-reflectance measurements on bulk specimens.
221 citations
TL;DR: In this article, the vibrational and optical properties of defect perovskites Cs2SnX6 (X = Cl, Br, I) and their use as hole-transporting materials (HTMs) in solar cells were reported.
Abstract: We report the vibrational and optical properties of the ‘defect’ perovskites Cs2SnX6 (X = Cl, Br, I) as well as their use as hole-transporting materials (HTMs) in solar cells. All three air-stable compounds were characterized using powder X-ray diffraction and Rietveld refinement. Far-IR reflectance, Raman, and UV–vis spectroscopy as well as electronic band structure calculations show that the compounds are direct band gap semiconductors with a pronounced effect of the halogen atom on the size of the energy gap and the vibrational frequencies. Scanning electron microscopy and atomic force microscopy confirmed that the morphology of the perovskite films deposited from N,N-dimethylformamide solutions on TiO2 substrates also strongly depends on the chemical composition of the materials. The Cs2SnX6 perovskites were introduced as hole-transporting materials in dye-sensitized solar cells, based on mesoporous titania electrodes sensitized with various organic and metal–organic dyes. The solar cells based on Cs2...
208 citations
TL;DR: The optoelectronic properties of CuInS2 nanocry crystals are described and what is known of their origin is discussed, and progress toward application of these "green" nanocrystals is summarized.
Abstract: The capacity of fluorescent colloidal semiconductor nanocrystals for commercial application has led to the development of nanocrystals with nontoxic constituent elements as replacements for the currently available Cd- and Pb-containing systems. CuInS2 is a good candidate material because of its direct band gap in the near-infrared spectral region and large optical absorption coefficient. The ternary nature, flexible stoichiometry, and different crystal structures of CuInS2 lead to a range of optoelectronic properties, which have been challenging to elucidate. In this Perspective, the optoelectronic properties of CuInS2 nanocrystals are described and what is known of their origin is discussed. We begin with an overview of their synthesis, structure, and mechanism of formation. A complete discussion of the tunable luminescence properties and the radiative decay mechanism of this system is then presented. Finally, progress toward application of these "green" nanocrystals is summarized.
199 citations
TL;DR: It is found that the addition of Cu adatoms can be used to controllably n-dope few layer black phosphorus, thereby lowering the threshold voltage for n-type conduction without degrading the transport properties.
Abstract: Few-layer black phosphorus is a monatomic two-dimensional crystal with a direct band gap that has high carrier mobility for both holes and electrons. Similarly to other layered atomic crystals, like graphene or layered transition metal dichalcogenides, the transport behavior of few-layer black phosphorus is sensitive to surface impurities, adsorbates, and adatoms. Here we study the effect of Cu adatoms onto few-layer black phosphorus by characterizing few-layer black phosphorus field effect devices and by performing first-principles calculations. We find that the addition of Cu adatoms can be used to controllably n-dope few layer black phosphorus, thereby lowering the threshold voltage for n-type conduction without degrading the transport properties. We demonstrate a scalable 2D material-based complementary inverter which utilizes a boron nitride gate dielectric, a graphite gate, and a single bP crystal for both the p- and n-channels. The inverter operates at matched input and output voltages, exhibits a ...
196 citations
TL;DR: In this article, the authors predicted that a novel 2D IV-VI material, namely germanium monosulfide (GeS) monolayer, is a rather desirable candidate.
Abstract: Low-dimensional semiconducting materials with moderate band gaps as well as high carrier mobilities are highly sought for future application in microelectronics. By means of hybrid density functional theory computations, we predicted that a novel two-dimensional (2D) IV–VI material, namely germanium monosulfide (GeS) monolayer, is a rather desirable candidate. According to our computations, GeS monolayer is semiconducting with an indirect band gap of 2.34 eV, which can be effectively tuned by employing an external strain. Remarkably, the GeS monolayer has an electron mobility of 3680 cm2 V−1 s−1, which is much higher than that of MoS2 monolayer. Our computations also revealed that GeS monolayer is a stable structure and can be obtained by exfoliation or mechanical cleavage techniques. These findings render GeS monolayer a promising 2D material for applications in future microelectronics, and call for more research attention on 2D IV–VI materials.
194 citations
TL;DR: The proposed two-dimensional material based on a single layer of violet or Hittorf's phosphorus is found to be energetically very stable, comparable to other previously proposed single-layered phosphorus structures, and to have a high and highly anisotropic hole mobility.
Abstract: We propose here a two-dimensional material based on a single layer of violet or Hittorf’s phosphorus. Using first-principles density functional theory, we find it to be energetically very stable, comparable to other previously proposed single-layered phosphorus structures. It requires only a small energetic cost of approximately 0.04 eV/atom to be created from its bulk structure, Hittorf’s phosphorus, or a binding energy of 0.3–0.4 J/m2 per layer, suggesting the possibility of exfoliation in experiments. We find single-layered Hittorf’s phosphorus to be a wide band gap semiconductor with a direct band gap of approximately 2.5 eV, and our calculations show it is expected to have a high and highly anisotropic hole mobility with an upper bound lying between 3000–7000 cm2 V–1 s–1. These combined properties make single-layered Hittorf’s phosphorus a very good candidate for future applications in a wide variety of technologies, in particular for high frequency electronics, and optoelectronic devices operating i...
194 citations
TL;DR: In this paper, the transition from an indirect to a fundamental direct bandgap material will be discussed, and the most commonly used approaches, i.e., molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), will be reviewed in terms of crucial process parameters, structural as well as optical quality and employed precursor combinations including Germanium hydrides, Silicon hydride and a variety of Sn compounds like SnD4, SnCl4 or C6H5SnD3.
TL;DR: In this paper, a systematically study on the CVD growth of continuous bilayer ReS2 film and single crystalline hexagonal ReS 2 flake, as well as their corresponding optoelectronic properties is reported.
Abstract: Rhenium disulfide (ReS2) is attracting more and more attention for its thickness-depended direct band gap. As a new appearing 2D transition metal dichalcogenide, the studies on synthesis method via chemical vapor deposition (CVD) is still rare. Here a systematically study on the CVD growth of continuous bilayer ReS2 film and single crystalline hexagonal ReS2 flake, as well as their corresponding optoelectronic properties is reported. Moreover, the growth mechanism has been proposed, accompanied with simulation study. High-performance photodetector based on ReS2 flake shows a high responsivity of 604 A·W−1, high external quantum efficiency of 1.50 × 105 %, and fast response time of 2 ms. ReS2 film-based photodetector exhibits weaker performance than the flake one; however, it still demonstrates a much faster response time (≈103 ms) than other reported CVD-grown ReS2-based photodetector (≈104–105 ms). Such good properties of ReS2 render it a promising future in 2D optoelectronics.
TL;DR: This work uses density functional theory and high field magneto-optics to investigate the metal chalcogenide InSe, a recent addition to the family of van der Waals layered crystals, which transforms from a direct to an indirect band gap semiconductor as the number of layers is reduced.
Abstract: The electronic band structure of van der Waals (vdW) layered crystals has properties that depend on the composition, thickness and stacking of the component layers. Here we use density functional theory and high field magneto-optics to investigate the metal chalcogenide InSe, a recent addition to the family of vdW layered crystals, which transforms from a direct to an indirect band gap semiconductor as the number of layers is reduced. We investigate this direct-to-indirect bandgap crossover, demonstrate a highly tuneable optical response from the near infrared to the visible spectrum with decreasing layer thickness down to 2 layers, and report quantum dot-like optical emissions distributed over a wide range of energy. Our analysis also indicates that electron and exciton effective masses are weakly dependent on the layer thickness and are significantly smaller than in other vdW crystals. These properties are unprecedented within the large family of vdW crystals and demonstrate the potential of InSe for electronic and photonic technologies.
Nanjing University1, University of California, Berkeley2, Rensselaer Polytechnic Institute3, Autonomous University of Madrid4, Lawrence Berkeley National Laboratory5, Geballe Laboratory for Advanced Materials6, University of Oxford7, Pohang University of Science and Technology8, SLAC National Accelerator Laboratory9
TL;DR: The finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.
Abstract: High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct–indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our findin...
TL;DR: In this article, the structural, electronic and optical properties of novel atomically thin systems based on germanene and antimonene nanocomposites have been investigated by means of density functional theory.
Abstract: In this work, the structural, electronic and optical properties of novel atomically thin systems based on germanene and antimonene nanocomposites have been investigated by means of density functional theory. We find that the germanene and antimonene monolayers are bound to each other via orbital hybridization with enhanced binding strength. Most importantly, the band gap opening can be achieved. Our results demonstrate that the AAII pattern has a direct band gap characteristic with a moderate value of up to 391 meV, while the other three patterns have indirect band gaps tunable from 37 to 171 meV. In particular, changing the direction and strength of the external electric field (E-field) can effectively tune the energy gap of the germanene/antimonene bilayer over a wide range even with a semiconductor–metal transition. The work function of the heterobilayer in the AAII pattern which possesses a direct band gap can be tinkered up from −3.21 to 12.33 eV by applying different E-field intensities. In addition, the germanene/antimonene bilayer exhibits more pronounced optical conductivity. The tunable bandgaps and work function together with a superior visible light response capability make the germanene/antimonene bilayer a viable candidate for optoelectronic applications.
TL;DR: In this paper, the geometric, energetic, and electronic properties of group IV-VI binary monolayers were investigated by employing density functional theory based electronic structure calculations, and the results of binding energy calculations showed that all the binary systems studied are energetically stable.
Abstract: We perform systematic investigation on the geometric, energetic, and electronic properties of group IV-VI binary monolayers $(\mathit{XY})$, which are the counterparts of phosphorene, by employing density functional theory based electronic structure calculations. For this purpose, we choose the binary systems $XY$ consisting of equal numbers of group IV $(X$ = C, Si, Ge, Sn) and group VI elements ($Y$ = O, S, Se, Te) in three geometrical configurations, the puckered, buckled and planar structures. The results of binding energy calculations show that all the binary systems studied are energetically stable. It is observed that, the puckered structure, similar to that of phosphorene, is the energetically most stable geometric configuration. Moreover, the binding energies of buckled configuration are very close to those of the puckered configuration. Our results of electronic band structure predict that puckered SiO and CSe are direct band semiconductors with gaps of 1.449 and 0.905 eV, respectively. Band structure of CSe closely resembles that of phosphorene. Remaining group IV-VI binary monolayers in the puckered configuration and all the buckled monolayers are also semiconductors, but with indirect band gaps. Importantly, we find that the difference between indirect and direct band gaps is very small for many puckered monolayers. Thus there is a possibility of making these systems undergo transition from indirect to direct band gap semiconducting state by a suitable external influence. Indeed, we show in the present work that seven binary monolayers, namely, SnS, SiSe, GeSe, SnSe, SiTe, GeTe, and SnTe become direct band gap semiconductors when they are subjected to a small mechanical strain ($\ensuremath{\le}3%$). This makes nine out of sixteen binary monolayers studied in the present work direct band gap semiconductors. Thus there is a possibility of utilizing these binary counterparts of phosphorene in future light-emitting diodes and solar cells.
TL;DR: In this article, the authors systematically studied four types of van der Waals heterostructures formed by monolayer graphene, h-BN, g-C3N4, and polyphenylene on ZrS2 nanosheets.
Abstract: As a fast emerging topic, van der Waals heterostructures can modify two-dimensional (2D) layered materials with desired properties, thus greatly extending the applications of these materials. Via state-of-the-art first-principles calculations, we systematically study four types of van der Waals heterostructures formed by monolayer graphene, h-BN, g-C3N4, and polyphenylene on ZrS2 nanosheets. A direct band gap can be obtained in the graphene/ZrS2 heterostructure, endowing graphene with the real ability to be applied in nanoelectronics, whereas the van der Waals interactions of graphene significantly broadens the optical absorption of ZrS2. The conduction band and valence band of the four heterostructures are contributed by the ZrS2 layer and the other layer, respectively, meaning good charge separation is achieved. We proposed that the strained h-BN/ZrS2 and g-C3N4/ZrS2 heterostructures satisfy fundamental aspects for photocatalytic water splitting, with the reduction and oxidation levels well inside their band gaps. By forming heterostructures with ZrS2, the optical properties of h-BN, g-C3N4 and polyphenylene show a remarkable improvement in the visible-light region. The findings in this study will be of broad interest in van der Waals heterostructure research and in the photocatalysis field.
TL;DR: It is shown that the photoluminescence (PL) emission can be selectively and reversibly engineered through energetically favorable electron transfer from photoexcited TMDCs to MPcs, further supporting the photoinduced charge transfer mechanism.
Abstract: Atomically thin transition metal dichalcogenides (TMDCs) have attracted great interest as a new class of two-dimensional (2D) direct band gap semiconducting materials. The controllable modulation of optical and electrical properties of TMDCs is of fundamental importance to enable a wide range of future optoelectronic devices. Here we demonstrate a modulation of the optoelectronic properties of 2D TMDCs, including MoS2, MoSe2, and WSe2, by interfacing them with two metal-centered phthalocyanine (MPc) molecules: nickel Pc (NiPc) and magnesium Pc (MgPc). We show that the photoluminescence (PL) emission can be selectively and reversibly engineered through energetically favorable electron transfer from photoexcited TMDCs to MPcs. NiPc molecules, whose reduction potential is positioned below the conduction band minima (CBM) of monolayer MoSe2 and WSe2, but is higher than that of MoS2, quench the PL signatures of MoSe2 and WSe2, but not MoS2. Similarly, MgPc quenches only WSe2, as its reduction potential is situ...
TL;DR: In this article, the effects of spin-orbit interaction, number of layers, and applied tensile strain on the vibrational and optical properties of single-layer, few-layer and bulk Molybdenum disulfide are analyzed from a theoretical point of view.
Abstract: Molybdenum disulfide, MoS2, has recently gained considerable attention as a layered material where neighboring layers are only weakly interacting and can easily slide against each other. Therefore, mechanical exfoliation allows the fabrication of single and multi-layers and opens the possibility to generate atomically thin crystals with outstanding properties. In contrast to graphene, it has an optical gap of 1.9 eV. This makes it a prominent candidate for transistor and opto-electronic applications. Single-layer MoS$_2$ exhibits remarkably different physical properties compared to bulk MoS$_2$ due to the absence of interlayer hybridization. For instance, while the band gap of bulk and multi-layer MoS$_2$ is indirect, it becomes direct with decreasing number of layers. In this review, we analyze from a theoretical point of view the electronic, optical, and vibrational properties of single-layer, few-layer and bulk MoS$_2$. In particular, we focus on the effects of spin-orbit interaction, number of layers, and applied tensile strain on the vibrational and optical properties. We examine the results obtained by different methodologies, mainly ab initio approaches. We also discuss which approximations are suitable for MoS$_2$ and layered materials. The effect of external strain on the band gap of single-layer MoS$_2$ and the crossover from indirect to direct band gap is investigated. We analyze the excitonic effects on the absorption spectra. The main features, such as the double peak at the absorption threshold and the high-energy exciton are presented. Furthermore, we report on the phonon dispersion relations of single-layer, few-layer and bulk MoS$_2$. Based on the latter, we explain the behavior of the Raman-active $A_{1g}$ and $E^1_{2g}$ modes as a function of the number of layers.
TL;DR: In this paper, the electronic structure of epitaxial single-layer Au(1) was investigated by angle-resolved photoemission spectroscopy, scanning tunneling spectrograph, and first-principles calculations.
Abstract: The electronic structure of epitaxial single-layer ${\mathrm{MoS}}_{2}$ on Au(111) is investigated by angle-resolved photoemission spectroscopy, scanning tunneling spectroscopy, and first-principles calculations. While the band dispersion of the supported single layer is close to a free-standing layer in the vicinity of the valence-band maximum at $\overline{K}$ and the calculated electronic band gap on Au(111) is similar to that calculated for the free-standing layer, significant modifications to the band structure are observed at other points of the two-dimensional Brillouin zone: at $\overline{\mathrm{\ensuremath{\Gamma}}}$, the valence-band maximum has a significantly higher binding energy than in the free ${\mathrm{MoS}}_{2}$ layer and the expected spin-degeneracy of the uppermost valence band at the $\overline{M}$ point cannot be observed. These band structure changes are reproduced by the calculations and can be explained by the detailed interaction of the out-of-plane ${\mathrm{MoS}}_{2}$ orbitals with the substrate.
TL;DR: The results suggest that the 2D MA2Pb(SCN)2I2 perovskite may not be among the most promising absorbers for efficient single-junction solar cell applications; however, use as an absorber for the top cell of a tandem solar cell may still be a possibility if films are grown with the 2 D layers aligned perpendicular to the substrates.
Abstract: We explore the photovoltaic-relevant properties of the 2D MA2Pb(SCN)2I2 (where MA = CH3NH3(+)) perovskite using a combination of materials synthesis, characterization and density functional theory calculation, and determine electronic properties of MA2Pb(SCN)2I2 that are significantly different from those previously reported in literature. The layered perovskite with mixed-anions exhibits an indirect bandgap of ∼2.04 eV, with a slightly larger direct bandgap of ∼2.11 eV. The carriers (both electrons and holes) are also found to be confined within the 2D layers. Our results suggest that the 2D MA2Pb(SCN)2I2 perovskite may not be among the most promising absorbers for efficient single-junction solar cell applications; however, use as an absorber for the top cell of a tandem solar cell may still be a possibility if films are grown with the 2D layers aligned perpendicular to the substrates.
TL;DR: In this paper, a class of two-dimensional SbAs honeycomb binary compounds is proposed, which can be tuned from semiconductor to topological insulator, and it is shown that the topological invariance of these compounds is 1.
Abstract: A stibarsen [derived from Latin stibium (antimony) and arsenic] or allemontite, is a natural form of arsenic antimonide (SbAs) with the same layered structure as arsenic and antimony. Thus, exploring the two-dimensional SbAs nanosheets is of great importance to gain insights into the properties of group V-V compounds at the atomic scale. Here, we propose a class of two-dimensional V-V honeycomb binary compounds, SbAs monolayers, which can be tuned from semiconductor to topological insulator. By ab initio density functional theory, both \ensuremath{\alpha}-SbAs and \ensuremath{\gamma}-SbAs display a significant direct band gap, while others are indirect semiconductors. Interestingly, in an atomically thin \ensuremath{\beta}-SbAs polymorph, spin-orbital coupling is significant, which reduces its band gap by 200 meV. Especially under biaxial tensile strain, the gap of \ensuremath{\beta}-SbAs can be closed and reopened with concomitant change of band shapes, which is reminiscent of band inversion known in many topological insulators. In addition, we find that the ${Z}_{2}$ topological invariant is 1 for \ensuremath{\beta}-SbAs under the tensile strain of 12%, and the nontrivial topological feature of \ensuremath{\beta}-SbAs is also confirmed by the gapless edge states which cross linearly at the \ensuremath{\Gamma} point. These ultrathin group-V-V semiconductors with outstanding properties are highly favorable for applications in alternative optoelectronic and quantum spin Hall devices.
TL;DR: In this article, a 2D semiconductor of SnS layers is proposed, which has an indirect band gap that can be tuned from 1.96 eV to 1.44 eV for a six-layer structure and the decrease of the band gap with increasing number of layers is not monotonic but shows an odd even quantum confinement effect.
Abstract: As a compound analogue of black phosphorus, a new 2D semiconductor of SnS layers is proposed. Based on state-of-the-art theoretical calculations, we confirm that such 2D SnS layers are thermally and dynamically stable and can be mechanically exoliated from α-phase SnS bulk materials. The 2D SnS layer has an indirect band gap that can be tuned from 1.96 eV for the monolayer to 1.44 eV for a six-layer structure. Interestingly, the decrease of the band gap with increasing number of layers is not monotonic but shows an odd–even quantum confinement effect, because the interplay of spin–orbit coupling and lack of inversion symmetry in odd-numbered layer structures results in anisotropic spin splitting of the energy bands. It was also found that such 2D SnS layers show high in-plane anisotropy and high carrier mobility (tens of thousands of cm2 V–1 s–1) even superior to that of black phosphorus, which is dominated by electrons. With these intriguing electronic properties, such 2D SnS layers are expected to have ...
TL;DR: In this paper, density functional calculations aimed at identifying the atomistic and electronic structure origin of the valence and conduction band, and band gap tunability of halide perovskites ABX3 upon variations of the monovalent and bivalent cations A and B and the halide anion X were performed.
Abstract: We performed density functional calculations aimed at identifying the atomistic and electronic structure origin of the valence and conduction band, and band gap tunability of halide perovskites ABX3 upon variations of the monovalent and bivalent cations A and B and the halide anion X. We found that the two key ingredients are the overlap between atomic orbitals of the bivalent cation and the halide anion, and the electronic charge on the metal center. In particular, lower gaps are associated with higher negative antibonding overlap of the states at the valence band maximum (VBM), and higher charge on the bivalent cation in the states at the conduction band minimum (CBM). Both VBM orbital overlap and CBM charge on the metal ion can be tuned over a wide range by changes in the chemical nature of A, B and X, as well as by variations of the crystal structure. On the basis of our results, we provide some practical rules to tune the valence band maximum, respectively the conduction band minimum, and thus the band gap in this class of materials.
TL;DR: In this paper, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments.
Abstract: Cesium and methylammonium bismuth iodides (Cs3Bi2I9 and MA3Bi2I9) are new low-toxic and air stable compounds in the perovskite solar cell family with promising characteristics. Here, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments. The calculated direct and indirect band gap values for Cs3Bi2I9 (2.17 and 2.0 eV) and MA3Bi2I9 (2.17 and 1.97 eV) are found to be in good agreement with the experimental optical band gaps (2.2, 2.0 eV and 2.4, 2.1 eV for Cs3Bi2I9 and MA3Bi2I9, respectively) estimated for solution-processed films. There is an error cancelation in the DFT calculated band gap similar to that for lead perovskites. However, fully relativistic DFT calculations indicate that the size of the spin orbit coupling (SOC) error cancelation for bismuth perovskite (0.5 eV) is less than for lead perovskite (1 eV), and other factors...
TL;DR: In this article, the authors showed that controlled heating in air significantly improves device performance of WSe2 FETs in terms of on-state currents and field-effect mobilities.
Abstract: Monolayer WSe2 is a two-dimensional (2D) semiconductor with a direct band gap, and it has been recently explored as a promising material for electronics and optoelectronics. Low field-effect mobility is the main constraint preventing WSe2 from becoming one of the competing channel materials for field-effect transistors (FETs). Recent results have demonstrated that chemical treatments can modify the electrical properties of transition metal dichalcogenides (TMDCs), including MoS2 and WSe2. Here, we report that controlled heating in air significantly improves device performance of WSe2 FETs in terms of on-state currents and field-effect mobilities. Specifically, after being heated at optimized conditions, chemical vapor deposition grown monolayer WSe2 FETs showed an average FET mobility of 31 cm2·V–1·s–1 and on/off current ratios up to 5 × 108. For few-layer WSe2 FETs, after the same treatment applied, we achieved a high mobility up to 92 cm2·V–1·s–1. These values are significantly higher than FETs fabricat...
TL;DR: In this article, the cubic phase of tin sulfide π-SnS was synthesized and compared to the α-snS phase, which is more stable than the ideal ideal rocksalt structure of SnS.
Abstract: We report on the synthesis of the newly discovered cubic phase of tin sulfide π-SnS and compare its properties to the well-known phase of tin sulfide, α-SnS. Shape control was achieved by the variation of synthesis parameters, resulting in cubic, rhombic dodecahedral and tetrahedral shapes of the π-SnS nanoparticles. X-ray diffraction provided authentication of the proposed model and refined determination of the lattice parameter a = 11.595 A. Raman spectroscopy showed a substantial shift towards higher energies and peak splitting for π-SnS. Optical absorption spectroscopy indicated an indirect band gap of 1.53 eV, in good agreement with density functional theory (DFT) calculations indicating a band gap greater than that of α-SnS. DFT total energy calculations show that the π-SnS phase is energetically similar to α-SnS, and is significantly more stable than the hypothetical ideal rocksalt structure of SnS.
TL;DR: In this paper, the phononic, electronic and optical properties of monolayer arsenene/antimonene were investigated by considering phonon dispersions, and the obtained electronic structures reveal the direct band gap and indirect band gap.
Abstract: Recently a stable monolayer of antimony in buckled honeycomb structure called antimonene was successfully grown on 3D topological insulator Bi$_2$Te$_3$ and Sb$_2$Te$_3$, which displays semiconducting properties. By first principle calculations, we systematically investigate the phononic, electronic and optical properties of $\alpha-$ and $\beta-$ allotropes of monolayer arsenene/antimonene. We investigate the dynamical stabilities of these four materials by considering the phonon dispersions. The obtained electronic structures reveal the direct band gap of monolayer $\alpha-$As/Sb and indirect band gap of $\beta-$As/Sb. Significant absorption is observed in $\alpha-$Sb, which can be used as a broad saturable absorber.
TL;DR: Zeraati et al. as mentioned in this paper investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb struc- ture of arsenic, using ab initio calculations.
Abstract: Highly Anisotropic Thermal Conductivity of Arsenene: an ab initio Study Majid Zeraati, 1 S. Mehdi Vaez Allaei, 1, 2, ∗ I. Abdolhosseini Sarsari, 3 Mahdi Pourfath, 4, 5 and Davide Donadio 6, 7, 8, 9, † Department of Physics, University of Tehran, Tehran 14395-547, Iran School of Physics, Institute for Research in Fundamental Sciences (IPM), Tehran 19395-5531, Iran Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran School of Electrical and Computer Engineering, University of Tehran, Tehran 14395-515, Iran Institute for Microelectronics, TU Wien, Gushausstrase 27–29/E360, 1040 Vienna, Austria Department of Chemistry, University of California Davis, One Shields Ave. Davis, CA, 95616 Max Planck Institut f¨ ur Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany Donostia International Physics Center, Paseo Manuel de Lardizabal, 4, 20018 Donostia-San Sebastian, Spain IKERBASQUE, Basque Foundation for Science, E-48011 Bilbao, Spain (Dated:) Elemental 2D materials exhibit intriguing heat transport and phononic properties. Here we have investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb struc- ture of arsenic, using ab initio calculations. Solving the Boltzmann transport equation for phonons, we predict a highly anisotropic thermal conductivity, of 30.4 and 7.8 W/mK along the zigzag and armchair directions, respectively at room temperature. Our calculations reveal that phonons with mean free paths between 20 nm and 1 µm provide the main contribution to the large thermal con- ductivity in the zigzag direction, mean free paths of phonons contributing to heat transport in the armchair directions range between 20 and 100 nm. The obtained anisotropic thermal conductivity and feasibility of synthesis, in addition to high electron mobility reported elsewhere, make arsenene a promising material for nano-electronic applications and thermal management. I. INTRODUCTION The discovery of graphene as a stable atomically thin material has led to extensive investigation of similar 2D systems. Its properties such as high electron mobility 1 , and very high thermal conductivity 2–5 make graphene very appealing for applications in electronics, for packag- ing and thermal management 6–11 . The successful iso- lation of single-layer graphene fostered the search for further ultra-thin 2D structures, such as silicene, ger- manene, phosphorene, and transition metal dichalco- genides, e.g. MoS 2 and WS 2 12,13 . These materials are now considered for various practical usages due to their distinguished properties stemming from their low dimen- sionality. Thermal transport in 2D materials has recently attracted the attention of the scientific community, as anomalous heat conduction has been predicted to oc- cur in systems with reduced dimensionality 14 . Phononic properties and thermal conductivity vary significantly from one 2D system to another 5,15–18 . For example, sil- icene has a buckled structure and a much lower thermal conductivity 19,20 compared to graphene 12,21,22 . 2D structures of arsenic and phosphorous have been recently investigated 23–27 . Arsenic and phosphorus are in the 5th group of the periodic table and both have different allotropes. Black phosphorus is a layered al- lotrope of phosphorus similar to graphite, and the stabil- ity of its single layer form, named phosphorene has been probed both theoretically and experimentally 13,28 . Gray arsenic is one of the most stable allotropes of arsenic with a buckled layered structure 27,29 . In addition, arsenic has an orthorhombic phase (puckered) similar to black phosphorus 23,25,26 , and its monolayer is called arsenene (see Fig. 1). Experimental observations have shown that gray arsenic undergoes a structural phase transition to the orthorhombic precursor of arsenene at temperatures of about T = 370 K 30 . As a monolayer arsenene can have a direct band gap of the order of 1 eV, as opposed to the multilayer allotrope, which exhibits an indirect band gap 23,26 . According to our calculations, arsenene is stable as a puckered monolayer also near zero temper- ature, in agreement with previous reports 23,25,26 . Both phosphorene and arsenene exhibit diverse polymorphs, with electronic properties that vary from semiconduct- ing to semimetallic and metallic as a function of struc- ture and strain 23,27,31 . Low dimensionality and the versa- tility of their electronic structure make two-dimensional 5th group elemental systems very appealing, not only for fundamental studies, but also for practical applications in nano electronics. For the latter applications, it is how- ever essential to characterize thermal transport and heat dissipation, to predict their operating temperatures. In this Article we investigate heat conduction in ar- senene and we elucidate the anisotropy of its thermal con- ductivity. We consider “puckered” unstrained arsenene, which is a semiconductor 23 . Since in semiconductors phonons are the predominant heat carriers, we use first- principles anharmonic lattice dynamics calculations and the Boltzmann transport equation to compute phonon dispersion relations and thermal conductivity. Accord- ing to electronic structure calculations 23 , arsenene has a band gap of the order of 1 eV, thus we expect a negligible electronic contribution to κ, unless the system is doped or strained. For this reason we restrict our study to the phononic contribution to thermal transport.
TL;DR: In this paper, the spin-polarized density functional theory was used to analyze the atomic structure, stability, energy, and mechanical and electronic properties of single-layer structures of arsenene.
Abstract: Using first-principles spin-polarized density functional theory, we carried out an analysis on the atomic structure, stability, energetics, and mechanical and electronic properties of single-layer structures of arsenene. These are buckled honeycomb, symmetric, and asymmetric washboard arsenene structures. Our analysis is extended to include layered three-dimensional crystalline phase of arsenic, as well as bilayer and trilayer structures to reveal dimensionality effects. The buckled honeycomb and symmetric washboard structures are shown to maintain their stability at high temperatures even when they are freestanding. As a manifestation of the confinement effect, the large fundamental band gap of single-layer phases decreases with increasing number of layers and eventually is closed. Concomitantly, lattice constants partially increase, while interlayer distances decrease. The effects of hybrid functional or self-energy corrections together with the spin-orbit coupling on the electronic structure of arsenene are crucial. The responses of direct and indirect band gaps to biaxial or uniaxial strain are rather complex and directional; while the fundamental band gap decreases and changes from indirect to direct with the biaxial strain applied to buckled arsenene, these effects are strongly directional for washboard arsenene. The width and the indirect/direct character of the band gap can be tuned by the number of layers, as well as by applied uniaxial/biaxial strain.