Indirect Band Gap Semiconductor Research Articles

First-principles study of surface modification of CuSe

Original bulk phases of two-dimensional atomic crystal materials are layered. However, a few relevant researches show that some of two-dimensional material crystals have non-layered bulk phases. In this work we investigate monolayer CuSe which is non-layered, belonging in a new kind of honeycomb graphene analogue. Monolayer CuSe is not suitable for application in electronic devices because of its metallic nature. In order to find new two-dimensional atomic crystal materials with excellent performance suitable for application in electronic devices, we change CuSe from metal to semiconductor through external atom modification. The first principles study of density functional theory is conducted to ascertain the energy band structure of monolayer CuSe after second periodic atoms have been added to the top, center and bridge sites. The characteristics of monolayer CuSe with addition of Li or B atoms are studied, including energy band structure, the density of states, differential charge density, and crystal orbital Hamiltonian population. The results show that after adding Li atoms to CuSe, the CuSe transforms from metallic to semiconductive property at all three positions, and Li atom is more easily to be modified in the hexagonal center of CuSe, with band gap being about 1.77 eV, the Fermi level biased towards the top of the valence band. The CuSe with addition of Li atoms exhibits a p-type semiconductor property, so it is a direct bandgap semiconductor. Adding B atom to the top of Cu atom can also make CuSe semiconductive, with a band gap of about 1.2 eV, the conduction band minimum at the <i>K</i> point, and the valence band maximum at the <i>Γ</i> point. The CuSe with addition of B atoms belongs in an indirect band gap semiconductor, and the Fermi energy level is biased towards the conduction band minimum, exhibiting the characteristics of an n-type semiconductor. According to the results of differential charge density and crystal orbital Hamiltonian population, the B atom is bound to the top of the monolayer CuSe with the B-Se polar covalent bond. The first principle study reveals the realization of metal-to-semiconductor transition from monolayer CuSe to Cu<i>X</i>Se (<i>X</i> = Li, B), and the calculation results also show that CuSe with addition of Li atoms or B atoms is likely to be used in future electronic devices.

Effect of strain and electric field on electronic structure and optical properties of Ga<sub>2</sub>SeTe/In<sub>2</sub>Se<sub>3</sub> heterojunction

Stacking two-dimensional materials into heterogeneous structures is an effective strategy to regulate their physical properties and enrich their applications in modern nanoelectronics. The electronic structure and optical properties of a new two-dimensional Janus Ga<sub>2</sub>SeTe/In<sub>2</sub>Se<sub>3</sub> heterojunction with four stacked configurations are investigated by first principles calculations. The heterojunction of the four configurations is an indirect band-gap semiconductor with a type-II band structure, and the photoelectron donor and acceptor materials are determined by the polarization direction of two-dimensional In<sub>2</sub>Se<sub>3</sub>. The light absorption rises to 25% in the visible region, which is conducive to the effective utilization of the solar visible light. The biaxial strain can induce direct-indirect bandgap transition, and the applied electric field can effectively regulate the bandgap of heterogeneous structure. The bandgap of AA2 configuration increases monotonically from 0.195 eV to 0.714 eV, but that of AB2 configuration decreases monotonically from 0.859 eV to 0.058 eV. The band of the heterojunction always maintains the type-II structure under the two kinds of configurations. The heterojunctions under compressive strain show better light absorption capability in the visible region with shorter wavelength. These results reveal the regulatory mechanism of the Janus Ga<sub>2</sub>SeTe/In<sub>2</sub>Se<sub>3</sub> van der Waals heterojunction electronic structure and provide theoretical guidance in designing novel optoelectronic devices.

Structural stability, electronic properties, and physical modulation effects of armchair-edged C<sub>3</sub>B nanoribbons

C<sub>3</sub>B monolayer is a typical graphene-like two-dimensional material, which has been successfully prepared experimentally. Here, we use the density functional theory to study the structural stability, electronic properties and physical regulation effects of its armchair-edged nanoribbons. The results show that for the bare-edged nanoribbons, if the ribbon edges are composed of C atoms completely (AA-type), their electronic phase is a semiconductor; when both ribbons edges consist of C and B atoms (BB-type) jointly, their electronic phase is a metal; if one edge of the ribbon is composed of C atoms and the other edge is comprised of B and C atoms (AB-type), their electronic phase is also a metal. This suggests that the B atom located at the ribbon edge plays an important role in determining whether nanoribbons is a metal or semiconductor. While for the H-terminated nanoribbons, they are all direct or indirect band-gap semiconductors. The carrier mobility of H-terminated nanoribbons is generally lower than that of bare-edged nanoribbon, which is closely related to their larger effective mass and higher deformation potential values. Meanwhile, it is found that the semiconducting nanoribbons are very sensitive to physical regulation, especially under applied compressive strain and external electric field, the band gap of the nanoribbons becomes very smaller, which is favorable for the absorption of light energy and development of novel optical devices.

Janus 2H-MXTe (M = Zr, Hf; X = S, Se) monolayers with outstanding thermoelectric properties and low lattice thermal conductivities.

Two-dimensional (2D) materials have been one of the most popular objects in the research field of thermoelectric (TE) materials and have attracted substantial attention in recent years. Inspired by the synthesized 2H-MoSSe and numerous theoretical studies, we systematically investigated the electronic, thermal, and TE properties of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers by using first-principles calculations. The phonon dispersion curves and AIMD simulations confirm the thermodynamic stabilities. Moreover, Janus 2H-MXTe were evaluated as indirect band-gap semiconductors with band gaps ranging from 0.56 to 0.90 eV using the HSE06 + SOC method. To evaluate the TE performance, firstly, we calculated the temperature-dependent carrier relaxation time with acoustic phonon scattering τac, impurity scattering τimp, and polarized scattering τpol. Secondly, the calculation of lattice thermal conductivity (κl) shows that these monolayers possess relatively poor κl with values of 3.4-5.4 W mK-1 at 300 K, which is caused by the low phonon lifetime and group velocity. After computing the electronic transport properties, we found that the n-type doped Janus 2H-MXTe monolayers exhibit a high Seebeck coefficient exceeding 200 μV K-1 at 300 K, resulting in a high TE power factor. Eventually, combining the electrical and thermal conductivities, the optimal dimensionless figure of merit (zT) at 300 K (900 K) can be obtained, which is 0.94 (3.63), 0.51 (2.57), 0.64 (2.72), and 0.50 (1.98) for n-type doping of ZrSeTe, HfSeTe, ZeSTe, and HfSTe monolayers. Particularly, the ZrSeTe monolayer shows the best TE performance with the maximal zT value. These results indicate the excellent application potential of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers in TE materials.

Photogalvanic effects in Janus monolayer In2SSe with vacancy defects

Linear photogalvanic effects (PGEs) in Janus monolayer In 2 SSe with five different central regions, which are pure, S-vacancy, In-vacancy, SSe-vacancy and InIn-vacancy are investigated by non-equilibrium Green's function technique combined with density functional theory. Janus monolayer In 2 SSe is a kind of new two-dimensional (2D) indirect bandgap semiconductor that generates photoresponse in the absence of external bias voltage, showing potential applications in low power consumption photoelectronic devices. It is found that the photoresponse is related to the polarization angle in the form of cosine function. All the introduction of defects has enhanced the photocurrent, but the degree is different. The concentration and the position of vacancy defects also affect the photoresponse. Compared with other vacancy types, single In vacancy has the best performance because it has the highest photocurrent (29.80) and the largest ER (183.33) simultaneously. Our results indicate that appropriate vacancy defects can significantly enhance the photoresponse and ER. Janus monolayer In 2 SSe is promising in photoelectric detection. • The photoresponse is related to the polarization angle in the form of cosine function. • Indium vacancy defects can significantly enhance the photoresponse and ER. • The concentration of vacancy defects also affects the photoresponse.

First principles study of electronic and optical properties of S-type heterostructures MoSi<sub>2</sub>N<sub>4</sub>/GeC

In this article, the first principles calculation method is used to study the MoSi<sub>2</sub>N<sub>4</sub>/GeC heterostructures, and calculate its structural, electronic, and optical properties. And the effects of different biaxial strains and vertical electric fields on the band structure and optical absorption characteristics of the heterostructures are also investigated. MoSi<sub>2</sub>N<sub>4</sub>/GeC heterostructure is an indirect bandgap semiconductor with a bandgap of 1.25 eV, with the built-in electric field direction pointing from the GeC layer to the MoSi<sub>2</sub>N<sub>4</sub> layer. In addition, its photogenerated carrier transfer mechanism conforms to the S-type heterostructures mechanism, thus improving the oxidation reduction potential of photocatalytic water decomposition, making it fully meet the requirements of photocatalytic water decomposition with pH = 0–14. Under biaxial strain, the band gap first increases and then decreases with the increase of compressive strain, and the light absorption performance in the ultraviolet region increases with compressive strain increasing. The band gap decreases as tensile strain increases, and the light absorption performance in the visible light region is enhanced in comparison with its counterpart under compressive strain. Under a vertical electric field, the band gap increases with positive electric field increasing, and decreases with negative electric field increasing. In summary, MoSi<sub>2</sub>N<sub>4</sub>/GeC heterostructures can be used as an efficient photocatalytic material in some fields such as optoelectronic devices and photocatalysis.

Theoretical insights into the defect performance of the wide bandgap semiconductor BaS.

Wide bandgap semiconductors materials (WBGSMs) are of great interest for their applications in transparent electronics and power electronics. Recent studies have shown that BaS is a potential transparent conducting material but the knowledge of it is deficient. Herein, we systemically investigate its electronic structure and evaluate the effects of its intrinsic defects and extrinsic dopants by utilizing the hybrid density functional method. The obtained results show that BaS is an indirect bandgap semiconductor with a bandgap of 3.88 eV. Its electron-effective mass is very small (0.33 m0). We find that the intrinsic n-type conductivity of BaS is connected with the shallow donor defect sulfur vacancy (VS). Regarding extrinsic dopants (group IA atoms), we find that Li and Na are favorable n-type dopants, while K and Rb are p-type dopants. Among these impurities, the Li interstitial (Liint) configuration possesses the lowest formation energy of 0.114 eV. Based on thermodynamic simulations, we find that the electron density can reach 2.39 × 1020 cm-3 in Li-doped BaS at room temperature, which is comparable to those of typical WBGSMs In2O3, BaSnO3, and β-Ga2O3. We expect BaS could replace typical WBGSMs in some applications. Moreover, its component elements Ba and S are non-toxic, cheap, and earth-abundant, making it a very competitive candidate for WBGSMs. Based on these results, we deem BaS a promising candidate for optoelectronic applications.

Janus GaOClX (X = F, Br, and I) monolayers as predicted using first-principles calculations: a novel class of nanodielectrics with superior energy storage properties.

Dielectric materials play an important role in devices for energy conversion and storage. Based on first-principles calculations, novel two-dimensional Janus GaOClX (X = F, Br, and I) monolayers with superior energy storage properties are predicted. They are indirect-bandgap semiconductors with bandgaps in the range of 2.18-4.36 eV, and possess anisotropic carrier mobility, strong mechanical flexibility, and excellent out-of-plane piezoelectricity. More importantly, it is found that the GaOCl monolayer and Janus GaOClX monolayers could exhibit an ultrahigh energy storage density (as high as 893.32 J cm-3) comparable to those of electrochemical supercapacitors and batteries, unparalleled by other dielectric materials reported to date. This work opens up a new window in searching for novel dielectric materials, which could be used in dielectric capacitors with superior energy storage density and power density, excellent efficiency and thermal stability.

Strain and electric field induced electronic property modifications in two-dimensional Janus SZrAZ2 (A = Si, Ge; Z = P, As) monolayers.

Recently, significant attention has been directed towards two-dimensional Janus materials owing to their unique structure and novel properties. In this work, we have introduced novel two-dimensional Janus monolayers, SZrAZ2 (A = Si, Ge; Z = P, As), through first principles. Our primary focus was the investigation of the controllable electronic properties exhibited by the Janus SZrAZ2 structures under the influence of strain and an external electric field. Our research findings indicate the dynamic and thermodynamic stability of Janus SZrAZ2 (A = Si, Ge; Z = P, As) monolayers. In the equilibrium state, these monolayers exhibit properties of an indirect band gap semiconductor. When subjected to biaxial strain and an external electric field, we observed that the dependency of SZrSiAs2 and SZrGeAs2 monolayers on an external electric field is very weak. Their electronic properties can only be modulated by applying biaxial strain. For SZrSiP2 and SZrGeP2 monolayers, their electronic properties can be modulated under biaxial strain and an external electric field, resulting in a transition from semiconducting to metallic behavior. Finally, we calculated the carrier mobility of these four structures and observed that the SZrGeAs2 monolayer exhibits a hole mobility of up to 597.52 cm2 s-1 V-1 in the x-direction, whereas the SZrSiP2 monolayer demonstrates an electron mobility of up to 479.30 cm2 s-1 V-1 in the y-direction. In the x-direction, the electron mobility of SZrSiAs2 and SZrGeP2 monolayers was measured to be 189.88 and 528.44 cm2 s-1 V-1, respectively. These values are greater than or equivalent to that of experimentally synthesized MoS2 (∼200 cm2 s-1 V-1). Our research lays the foundation for utilizing two-dimensional Janus materials in electronic devices.

A first-principles study of the electronic, optical, and transport properties of novel transition-metal dichalcogenides

The band structures confirmed an indirect band gap semiconductor nature. All the static dielectric constant values demonstrate a decreasing tendency with the energy gap value.

Oxygen functionalized InSe and TlTe two-dimensional materials: transition from tunable bandgap semiconductors to quantum spin Hall insulators.

From first-principles calculations, we found that oxygen functionalized InSe and TlTe two-dimensional materials undergo the following changes with the increased concentrations of oxygen coverage, transforming from indirect bandgap semiconductors to direct bandgap semiconductors with tunable bandgap, and finally becoming quantum spin hall insulators. The maximal nontrivial bandgap are 0.121 and 0.169 eV, respectively, which occur at 100% oxygen coverage and are suitable for applications at room temperature. In addition, the topological phases are derived from SOC induced p-p bandgap opening, which can be further determined by Z2 topological invariants and topologically protected gapless edge states. Significantly, the topological phases can be maintained in excess of 75% oxygen coverage and are robust against external strain, making the quantum spin hall effect easy to achieve experimentally. Thus, the oxygen functionalized InSe and TlTe are fine candidate materials for the design and fabrication of topological devices.

Theoretical prediction of valley spin splitting in two-dimensional Janus MSiGeZ4 (M = Cr and W; Z = N, P, and As).

With the exploration of valleytronic materials in MA2Z4 structures, larger valley spin splitting has become a hot topic of research. Based on first-principles calculations, we predicted six valleytronic 2D (two-dimensional) Janus MSiGeZ4 (M = Cr and W; Z = N, P, and As) materials. The valley spin splitting value of WSiGeZ4 (Z = N, P, and As) can reach more than 400 meV, which is favorable for the practical application of valleytronics. Two-dimensional WSiGeZ4 (Z = N, P, and As) materials are dynamically and mechanically stable and have an abundance of electronic properties. The two-dimensional Janus WSiGeZ4 (Z = N, P, and As) structures comprise both direct and indirect bandgap semiconductor materials. Among them, WSiGeN4 is an indirect bandgap semiconductor material with a bandgap of 1.654 eV and WSiGeP4 is a direct bandgap semiconductor material. The valley spin splitting originates from the symmetry breaking of the material structure and the spin-orbit coupling effect of the transition metal, which is manifested as the Berry curvature. In particular, the Berry curvature of 2D Janus WSiGeP4 and WSiGeAs4 is as high as 300 Bohr2, which is quite large. The W atom has more d-orbital electrons than the Cr atom, and the SOC (spin-orbit coupling) effect is stronger; thus, the valley spin splitting value CrSiGeZ4 of WSiGeZ4 is more than 300 meV, which is quite large. In addition, the bandgap and valley spin splitting of WSiGeZ4 (Z = N, P, and As) can be significantly modulated by applying a biaxial strain. Our study shows that WSiGeZ4 (Z = N, P, and As) has great potential for valleytronic applications.

Strain-induced phase transitions and high carrier mobility in two-dimensional Janus MGeSN2 (M = Ti, Zr, and Hf) structures: first-principles calculations.

In this study, we construct new 2D Janus MGeSN2 (M = Ti, Zr, and Hf) monolayers and systematically investigate their electronic band structures under applied biaxial strain. Their crystal lattice and electronic as well as transport properties are also examined based on the first-principles calculations and deformation potential theory. The results show that the MGeSN2 structures have good dynamical and thermal stability, and their elastic constants satisfy the criteria of Born-Huang also indicating the good mechanical stability of these materials for experimental synthesis. Our calculated results indicate that the TiGeSN2 monolayer exhibits indirect-bandgap semiconductor characteristics whereas the ZrGeSN2 and HfGeSN2 monolayers exhibit direct-bandgap semiconductor characteristics. Importantly, the biaxial strain shows significant influences on the electronic energy band structures of the monolayers in the presence of a phase transition from semiconductor to metal, which is an important feature of these materials for their application in electronic devices. All three structures exhibit anisotropic carrier mobility in both x and y transport directions, suggesting their great potential for application in electronic devices.

Augmented thermoelectric performance of LiCaX (X = As, Sb) Half Heusler compounds via carrier concentration optimization

The present study is focussed on the detailed physical insight into the structural, thermal, and dynamical properties of 8-valence electron Half Heusler (HH) compounds LiCaX (X = As, Sb) using Density functional theory. The thermal and dynamic stabilities of the compounds are assessed via ab-initio molecular dynamic simulations and phonon dispersion calculations, respectively. The Tran-Blaha modified Becke Johnson potential is used to accurately predict the band gap of investigated compounds. It is found that they are indirect band gap semiconductors with band gaps of 2.52 eV (LiCaAs) and 2.09 eV (LiCaSb). The transport parameters are obtained for p-type and n-type doping at temperatures ranging from 300 K to 800 K by solving the Boltzmann Transport equation. The deformation potential theory is employed to calculate the temperature dependent relaxation time for both compounds. The results of the various thermoelectric parameters obtained using actual values of time-dependent relaxation time are compared with that calculated under constant relaxation time approximation. The maximum power factor is 10.95 × 1011 (4.99 × 1011) Wm−1K−2s−1 for p-type (n-type) LiCaAs and 12.53 × 1011 (5.30 × 1011) Wm−1K−2s−1 for p-type (n-type) LiCaSb at optimized carrier concentration. The obtained low lattice thermal conductivities for LiCaSb (0.66 Wm−1K−1) and LiCaAs (0.88 Wm−1K−1) are explicated it in terms of different phonon modes. The Figure of Merit at 800 K for p-type (n-type) LiCaAs is as high as 0.90 (0.73) and 0.93 (0.84) for LiCaSb at optimum carrier concentration ∼1020 cm−3 (∼1019 cm−3), which has been experimentally realized in other 8-valence electron Li-based HH compounds. The good thermoelectric performance of p-type LiCaX in comparison to n-type suggests that p-type LiCaX alloys are viable candidates for high temperature energy harvesting applications.

Tuning the Electronic and Mechanical Properties of Kagome Graphene via Hydrogenation

Surface passivation is proved to be an effective way to adjust material properties or to explore new two-dimensional (2D) materials. Herein, we proposed three hydrocarbons with high stability for the first time via hydrogenation on the Kagome graphene, namely, C6H4, C6H6-I, and C6H6-II. Unlike the Kagome graphene, which is metallic, all these 2D monolayers are wide-bandgap semiconductors (4.06–4.81 eV). Among them, C6H4 is an indirect bandgap semiconductor, but both C6H6-I and C6H6-II possess the direct bandgap feature. Considerable carrier mobilities (102 to 103 cm2 V–1 s–1) have been further confirmed in the three hydrocarbons on the basis of modified deformation potential theory. Specifically, for C6H4, the hole mobilities are as high as 104 to 105 cm2 V–1 s–1, comparable to those of graphene and black phosphorus. The intrinsic vertical electric field induced by the asymmetric crystal structures in C6H4 and C6H6-I will be beneficial to the spatial separation of electrons and holes in semiconductors, promising in the field of optoelectronics. In addition, hydrogenation has a great influence on the mechanical properties of Kagome graphene, no matter whether it is Young’s modulus, Poisson’s ratio, or ideal tensile strength. Particularly, in-plane axial negative Poisson’s ratios (−0.011/–0.018 along the a-/b-direction) were found in C6H6-I, mainly originated from the interaction of carbon pentagons and octagons. These interesting findings in our work may pave the way for the application of hydrogenated Kagome graphene in the future.

Strain engineering the electronic properties of the type-II CdO/MoS2 van der Waals heterostructure

The structural stability, electronic properties, band edge alignment and photocatalytic water splitting characteristics of the CdO/MoS2 heterostructure have been systematically investigated by first-principles calculations. The results show that the CdO/MoS2 heterostructure is a stable indirect bandgap (1.35 eV) semiconductor with a type-II band alignment and a large built-in electric field pointing from MoS2 to CdO, which greatly hinders the recombination of photogenerated carriers. Meanwhile, the CdO/MoS2 heterostructure with large carrier mobility can withstand the large biaxial strains. The band edge positions of the CdO/MoS2 heterostructure can be modulated by biaxial strain engineering. These results provide a theoretical basis for the application of the CdO/MoS2 heterostructure in photovoltaic and other electronic devices.

Postproduction Approach to Enhance the External Quantum Efficiency for Red Light-Emitting Diodes Based on Silicon Nanocrystals.

Despite bulk crystals of silicon (Si) being indirect bandgap semiconductors, their quantum dots (QDs) exhibit the superior photoluminescence (PL) properties including high quantum yield (PLQY > 50%) and spectral tunability in a broad wavelength range. Nevertheless, their low optical absorbance character inhibits the bright emission from the SiQDs for phosphor-type light emitting diodes (LEDs). In contrast, a strong electroluminescence is potentially given by serving SiQDs as an emissive layer of current-driven LEDs with (Si-QLEDs) because the charged carriers are supplied from electrodes unlike absorption of light. Herein, we report that the external quantum efficiency (EQE) of Si-QLED was enhanced up to 12.2% by postproduction effect which induced by continuously applied voltage at 5 V for 9 h. The active layer consisted of SiQDs with a diameter of 2.0 nm. Observation of the cross-section of the multilayer QLEDs device revealed that the interparticle distance between adjacent SiQDs in the emissive layer is reduced to 0.95 nm from 1.54 nm by "post-electric-annealing". The shortened distance was effective in promoting charge injection into the emission layer, leading improvement of the EQE.

Effect of warpage on the electronic structure and optical properties of bilayer germanene

The electronic structure and optical properties of bilayer germanene under different warpages are studied by the first-principles method of density functional theory. The effects of warpages on the electronic structure and optical properties of bilayer germanene are analyzed. The results of the electronic structure study show that the bottom of the conduction band of bilayer germanene moves to the lower energy direction with the increase of warpages at the K point, and the top of the valence band stays constant at the K point, and so the band gap decreases with the increase of warpage. When the warpage is 0.075 nm, the top of the valence band of bilayer germanene changes from K point to G point, and the bilayer germanene becomes an indirect band gap semiconductor. This is an effective means to modulate the conversion of bilayer germanene between direct band gap semiconductor and indirect band gap semiconductor by adjusting the band structure of bilayer germanene effectively. The study of optical properties shows that the effect of warpage on the optical properties of bilayer germanene is mainly distributed in the ultraviolet and visible regions, and the warpage can effectively regulate the electronic structure and optical properties of bilayer germanene. When the warpage is 0.069 nm, the first peak of dielectric function and extinction coefficient is the largest, and the energy corresponding to the absorption band edge is the smallest. Therefore, the electron utilization rate is the best when the warpage is 0.069 nm.

Graphene-like silicon carbide layer for potential safe anode lithium ion battery: A first principle study

<h2>Abstract</h2> The persistent safety challenge accompanying the use of carbon as anode material for lithium-ion batteries is a major setback in its use for energy storage applications unless a suitable replacement is found. Here we investigate the structural, electronic and electrochemical properties of graphene-like Silicon Carbide (SiC) using density functional theory (DFT). The result shows that SiC is an indirect band gap semiconductor with a band gap energy of 2.35 eV. Strong adsorption of Li on SiC is also observed with a transition from semiconducting to metallic phase upon full lithiation. The SiC also retains its structure after adatom adsorption indicating good cycling stability. The calculated value of 1.5 eV for the open circuit voltage is comparable with those of TiO₂. At this value, the metal lithium deposition, dendrite formation and formation of solid electrolyte interface leading to battery short circuits and other battery problems should be avoided. The theoretical capacity at the maximum concentration is 122mAhg⁻¹, comparable to other two-dimensional (2D) materials. These findings suggest graphene-like SiC layer as a potential safe anode material for Li-ion batteries.

One step reactive ion etching of black germanium conical nanostructures: Ultra-wide solar spectral absorption, finite element simulation, super hydrophilicity, photothermal conversion

We report a reactive ion etching process based on SF 6 –CHF 3 , which can prepare black germanium at low RF power and normal temperature. The absorption of black germanium in the spectral range of 300–1750 nm is greater than 99%. Spectral selectivity optimization allows nano tapered germanium to absorb nearly 98.1% of solar radiation. This nanostructure is uniform in a large area and also supports patterning design for the application in related fields such as photothermal imaging. Here, we discuss the effect of SF 6 and CHF 3 plasma concentrations on the progress of chemical reactions on the germanium surface and their essential role in forming tapered nanostructures. We observe its surface plasmon resonance by finite-difference time-domain way. Finally evaluate its feasibility in devices by photothermal conversion and water droplet evaporation experiments. The proposal of this work provides an available method for semiconductor micromachining, and also provides experimental and theoretical support for the efficient thermal utilization of solar energy. In conclusion, we analyzed the mechanism of one-step etching of highly absorbing tapered nanostructures by SF 6 –CHF 3 -based RIE process by SEM and XPS. The germanium surface tends to form particles at the initial stage of etching, and due to the very high concentration of negative ions in the SF 6 plasma, these particles gradually form GeS x clusters. The germanium fluoride is mainly present at the surface of the germanium sample, while the germanium sulfide extends to a greater depth, so these clusters are not effective in preventing etching. The CHF 3 plasma greatly reduces the negative ion concentration and reduces the tendency of particle formation in favor of smoother GeS x and C x H z F y layers, which are very favorable for the formation of nanostructures. Aggregation of the germanium sidewalls decreases with depth, so tapered nanostructures are gradually formed with etch time. In addition, over-etching leads to additional carbon deposition, which is mainly composed of fluorocarbon polymers (C x H y F z ) formed from the decomposition products of CHF 3 . The wide spatial distribution of the sizes of the tapered nanostructures makes the absorptivity of black germanium more than 99% in the 300–1750 nm spectral range. As an indirect bandgap semiconductor, the absorption of photons makes electrons excited from the valence band to the conduction band with a great probability of releasing energy to the lattice, converting it into phonons, and turning it into heat energy. The cone-shaped nanostructured germanium can be heated to 69.4 °C in 20 min, and the heating rate can reach 7.98 °C/min in 300 s. The conical nanostructure modification makes germanium exhibit superhydrophilicity, and the water droplets make wet contact with germanium and fill the grooves on the surface of the nanostructures, making it capable of completely evaporating 0.05 ml droplets within 101 s. It is very competitive in heating and evaporation of water droplets.a) Black germanium photos, and SEM images of nano cones; b) Comparison of absorptivity between black germanium and polished germanium; c) Infrared image of graphic germanium wafer; (the letter part is nano cone structure, and the rest is polished germanium) d) With the increase of etching time, the temperature change curve of the sample under simulated sunlight irradiation (1 solar intensity). • Disordered nanocrystalline germanium was prepared by metal induced one-step self masked RIE. • The absorbance in the 300-1750 nm regions was higher than 98%. • The structure has excellent optical properties due to light trapping effect and plasma resonance. • The excellent optical properties and super hydrophilic properties make the structure have excellent photothermal properties.