Modification Of Band Structure Research Articles

Strain dependence of the thermoelectric properties of perovskite MgSiO3: A first principles study

Using first-principles density functional theory based approach, we investigate the tunability of the thermoelectric properties of perovskite MgSiO3. We find that the thermoelectric properties are strongly correlated with the electronic structure and can, therefore, be modulated with uni-axial tensile and compressive strains. Key thermoelectric parameters, such as the Seebeck coefficient (S), show non-monotonic behavior, displaying large shifts with the application of 1% uni-axial tensile strain. The relevance of this small strain value also appears in structural distortion of the oxygen octahedra and band convergence in the electronic band structure. The band structure modifications also introduce a strong strain dependence of the relaxation time, as defined within the deformation potential approach. As a result, an improvement of up to 35% is observed in zT with the application of 1% tensile strain. Our results highlight a possible new route to improving thermoelectric performance in perovskite oxide thermoelectrics.

Coating of gallium-based liquid metal particles with molybdenum oxide and oxysulfide for electronic band structure modulation.

Liquid metal (LM) droplets are now used in many applications including catalysis, sensing, and flexible electronics. Consequently, the introduction of methods for on-demand alternating electronic properties of LMs is necessary. The active surface of LMs provides a unique environment for spontaneous chemical reactions that enable the formation of thin layers of functional materials for such modulations. Here, we showed the deposition of n-type MoOx and MoOxSy semiconductors on the surface of EGaIn LM droplets under mechanical agitation to successfully modulate their electronic structures. The "liquid solution"-"liquid metal" interaction resulted in the formation of oxide and oxysulfide layers on the surface of LM droplets. The comprehensive study of electronic and optical properties revealed a decrease in the band gap of the droplets after surface decoration with MoOx and MoOxSy, leading to deeper n-type doping of the materials. This method provides a facile procedure for engineering the electronic band structure of LM-based composites when they are necessary for various applications.

Growth mechanism study and band structure modulation of a manganese doped two-dimensional BlueP-Au network.

Two-dimensional (2D) materials are a very promising material family. The two-dimensional inorganic metal network called BlueP-Au network is rapidly attracting the attention of researchers due to its customizable architecture, adjustable chemical functions and electronic properties. Herein, manganese (Mn) was successfully doped on a BlueP-Au network for the first time, then the doping mechanism and electronic structure evolution was studied by in situ X-ray photoelectron spectroscopy (XPS) based on synchrotron radiation, X-ray absorption spectroscopy (XAS), Scanning Tunneling Microscopy (STM), Density functional theory (DFT), Low-energy electron diffraction (LEED), Angle resolved photoemission spectroscopy (ARPES), etc. Mn atoms tend to be stably adsorbed on two sites of the BlueP-Au network. It was the first observation that atoms can absorb on the two sites stably simultaneously. It is different from the previous adsorption models of BlueP-Au networks. The band structure was also successfully modulated, and overall down about 0.25 eV relative to the Fermi edge. It provided a new strategy for customizing the functional structure of the BlueP-Au network, which has provided new insights into monatomic catalysis, energy storage and nano electronic devices.

Enhanced Thermoelectric Performance in GeTe by Synergy of Midgap state and Band Convergence

AbstractThe good co‐existence of midgap state and valence band degeneracy is realized in Bi‐alloyed GeTe through the In‐Cd codoping to play different but complementary roles in the valence band structure modification. In doping induces midgap state and results in a considerably improved Seebeck coefficient near room temperature, while Cd doping significantly increases the Seebeck coefficient in the mid‐high temperature region by promoting the valence band convergence. The synergistic effects obviously increase the density of state effective mass from 1.39 to 2.65 m0, and the corresponding carrier mobility still reaches 34.3 cm2 V−1 s−1 at room temperature. Moreover, the Bi‐In‐Cd co‐alloying introduces various phonon scattering centers including nanoprecipitates and strain field fluctuations and suppresses the lattice thermal conductivity to a rather low value of 0.56 W m−1 K−1 at 600 K. As a result, the Ge0.89Bi0.06In0.01Cd0.04Te sample obtains excellent thermoelectric properties of zTmax ≈2.12 at 650 K and zTavg ≈1.43 between 300 and 773 K. This study illustrates that the thermoelectric performance of GeTe can be optimized in a wide temperature range through the synergy of midgap state and valence band convergence.

Effect of chromium doping on superconductivity and charge density wave order in the kagome metal Cs(V1−xCrx)3Sb5

The recently discovered kagome metal $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\mathrm{K}$, Rb, Cs) with intertwined superconductivity and charge density wave (CDW) order has attracted considerable interest. Here we investigate the evolution of CDW and superconductivity in $\mathrm{Cs}{({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x})}_{3}{\mathrm{Sb}}_{5}$, where chromium substitution for vanadium introduces extra electron carriers. Our experiments reveal that the CDW order is gradually suppressed under chromium doping, which can be understood from the band structure modifications induced by electron doping. This highlights the importance of the van Hove singularities near the $M$ and $L$ points for CDW instability. The anomalous Hall effect that is coupled with CDW in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ only appears in lightly doped $\mathrm{Cs}{({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x})}_{3}{\mathrm{Sb}}_{5}$ and vanishes at $x\ensuremath{\ge}0.06$ when the CDW order remains relatively strong. Unlike hole- and isovalent-doped $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$, where superconductivity is enhanced upon suppression of CDW, superconductivity is rapidly suppressed under chromium doping, pointing to a conventional $s$-wave pairing.

Impact of defects on the χ(3) optical nonlinearity of sputtered WSe2 thin films in the optical communication band

We present an experimental investigation on third-order nonlinear optical properties of RF-sputtered WSe2 thin films using a single-beam Z-scan technique around the optical communication window in a near-infrared spectral band. The nonlinear absorption coefficient was obtained from the open-aperture Z-scan transmission using ultrashort pulses centered at 1520nm excitation wavelength. The results show a reverse saturable absorption signature for all the WSe2 films due to two-photon absorption (TPA) mediated electronic transitions. The dependency of the TPA coefficient on film thickness is analyzed through the modification in an electronic band structure essentially caused due to defects induced in the samples. In addition, we also explored the impact of high laser intensity on the TPA process, which essentially pointed toward a significant contribution of free-carrier absorption at small laser intensities in WSe2 thin films. This investigation provides a basis for optimally tailoring the nonlinear optical properties of transition-metal-dichalcogenides via bandgap engineering for improving photonic device functionality.

Fabricating metal-free Z-scheme heterostructures with nitrogen-deficient carbon nitride for fast photocatalytic removal of acetaminophen

Photocatalytic technology has been increasing explored for the elimination of pollutants from aquatic environments. However, constructing powerful photocatalyst to drive rapid decontamination remains challenging. Here, we assemble boron-doped nitrogen-deficient carbon nitride (BNCN) into metal-free Z-scheme heterojunction via a facile approach to boost photocatalytic degradation of acetaminophen. Incorporating boron dopants and nitrogen defects to C3N4 significantly increase the photoabsorption. Furthermore, the constructed heterojunction exhibited decreased band gap and remarkably enhanced separation efficiency of photogenerated carriers compared with individual BNCN. Owing to the modulation of band structures and the better separation of photogenerated carriers, acetaminophen conversion approaching 100 % was achieved in just 30 mins under simulated sunlight, outperforming equivalent quantities of bulk carbon nitride and commercial TiO2 by 3.1 and 2.7 times, respectively. Two possible photodegradation pathways were proposed with hydroxylation and ring-opening reactions as dominant degradation reactions. Our findings provide an instructive catalyst design strategy to boost photocatalytic degradation efficiency.

A tailored direct-to-indirect band structure transition in double perovskite oxides influences its photocatalysis efficiency

The increase in cost-effective materials for harvesting solar energy is essential for the development of renewable energy technologies. Photovoltaics and photocatalysts are been actively researched in the field of harvesting solar photons. In this communication, we explore stable Ru-doped Ba2HoSbO6 (BHSR) complex perovskites with promising photocatalytic properties and a bandgap that is tunable in the UV–Vis region. The band gap shows a non-linear decrease with increased Ru doping, which follows an asymmetrically-weighted Vegard's law. UV–Visible absorption measurements and density functional theory (DFT) based simulations reveal a transition in magnitude (energy) as well as nature of the band structure (direct-to-indirect) upon Ru substitution; an observation which is unique and can influence future band structure engineering in perovskite oxides. This tailored band structure engineering plays an important role in the carrier recombination process aptly captured through photoluminescence (PL) spectroscopy. The photocatalytic activity of BHSR is demonstrated through a time varying photo-degradation of rhodamine B (Rh–B)-particulate BHSR solution. The catalytic activity is seen to improve with increased Ru doping which can be attributed to the reduction in the electron localization (electrical conductivity), increased dielectric screening, decrease in band gap and carrier recombination (electronic structure).

Activation of peroxymonosulfate by S-scheme Bi2S3/doped gCN heterostructure photocatalyst for highly efficient visible light driven tetracycline degradation: Insights into reaction mechanisms

A solid state microwave (MW) assisted hydrothermal method was used to prepare S-scheme heterojunction composite Bi2S3@doped gCN (BS@CN). TC was degraded upto 99.9 % under 20 min visible light, with synergistic action of 0.2 g/L catalyst having 10 wt% Bi2S3 (BS(10)@CN) and 1.5 g/L PMS, for initial TC concentration of 50 mg/L, exhibiting first order rate constant (kapp): 0.215 min−1. The modulation of band-structure due to the generation of heterojunction, coupled with different surface-bound redox cycles, i.e.,Co2 +/Co3 +surf.,Ce3 +/Ce4 +surf. andBi3 +/Bi4 +surf., helped the charge migration, separation of excited e-/h+ and production of reactive species, like,SO4∙-,∙OH,O2∙-,O21, etc., that played important roles towards TC degradation. Band-structure determination, XPS analysis and other characterizations revealed the formation of S-scheme configuration and corresponding surface-bound metastable complexes towards self-regeneration and excellent catalytic activity of the composite, even in the contaminated real-life surface water sources. Extensive intermediate analysis was carried out to develop a detailed TC degradation pathway. The BS@CN photocatalyst can be recycled multiple times, without significant loss of photocatalytic activity (∼88 % TC degradation after 5 cycles). This study demonstrated the design and fabrication of metal oxide doped gCN/metal chalcogenide-based heterojunction photocatalysts that have potential applications towards the removal of refractory contaminants in aqueous medium.

Strong redox-capable graphdiyne-based double S-scheme heterojunction 10%GC/Mo for enhanced photocatalytic hydrogen evolution

Graphdiyne (GDY) is a new two-dimensional layered structure of carbon isomers composed of sp and sp2 hybridized carbon atoms, which has attracted much attention in many fields because of its unique energy band structure, stability and high carrier mobility. However, GDY has been rarely reported in the study of photocatalytic hydrogen precipitation. In this work, graphdiyne-CuI (GDY-CuI) was successfully prepared by cross-coupling method using Cu salt (CuI) instead of Cu foil. Based on the modulation of the energy band structure and redox ability of the photocatalyst, a typical S-scheme heterojunction was constructed by introducing MoO2 through in situ sonication and stirring, and the composite catalyst 10%GC/Mo exhibited step-like electron transfer and strong redox ability. Under the action of internal electric field, this unique heterojunction can induce rapid electron transfer in a directional manner, which effectively promotes the separation of photogenerated electrons and holes in MoO2, thus improving the photocatalytic hydrogen evolution performance. The photocatalytic hydrogen evolution rate of the composite photocatalyst 10%GC/Mo was 32.8 μmol (8 mg of catalyst acted for 5 h), which was 32 times that of primitive MoO2. The introduction of GDY-CuI accelerates the charge separation and transfer rate, prolongs the lifetime of photogenerated carriers, and reduces the reaction overpotential. This work provides a reference for the application of the novel two-dimensional photocatalytic materials GDY and GDY-CuI in the field of photocatalysis.

Pulsed laser induced synthesis of graphitic carbon nitride-cadmium selenide nanocomposite for photo-catalytic degradation of organic dyes, and electro-catalytic hydrogen evolution reaction

Graphitic carbon nitride (g-C3N4) is a visible light active photo-catalyst, but its photo-catalytic efficiency is hampered by rapid recombination of the photo-induced charge carriers in g-C3N4. This adverse charge recombination of g-C3N4 is efficiently retarded by compositing g-C3N4 with cadmium selenide (CdSe). The energy band gap of CdSe is narrower than that of g-C3N4, and also its band positions are compatible with g-C3N4 in order to carry out the lateral intersystem crossing of the photo-induced electrons from the conduction band of g-C3N4 to that of CdSe. In this work, g-C3N4/CdSe nanocomposites with four different CdSe mass loading (1%, 2.5%, 5%, and 10%) were synthesized by pulsed laser induced method, where laser fragmentation (LFL), and laser defect engineering (LDL) are the underlying processes. The structural, morphological, and elemental characterizations using XRD, TEM, FE-SEM, and EDX confirmed the proper anchoring of CdSe nanoparticles on the g-C3N4 polymeric surface. The modified band structure in the g-C3N4/CdSe nanocomposite not only enhanced the visible light absorption, but also effectively retarded the photo-induced charge recombination, which are reflected in the optical absorption spectra, and the photoluminescence spectra respectively. Photo-catalytic degradations of Rhodamine B (RhB), and methylene blue (MB) dyes under visible light in the presence of four different variants of g-C3N4/CdSe nanocomposites were estimated, and bench marked with pure g-C3N4. As anticipated, the photo-catalytic degradation efficiency of all the variants of g-C3N4/CdSe nanocomposites is significantly higher than that with pure g-C3N4. The highest photo-catalytic degradation rate constant of 0.0222 min−1, 0.0178 min−1 respectively for MB and RhB were achieved using g-C3N4/CdSe nanocomposite with 2.5% CdSe mass content. As a second application, g-C3N4/CdSe nanocomposite was also used in the electrocatalytic hydrogen evolution process in the acidic medium. Thin this study, the observed over potential recorded was lower for g-C3N4/CdSe nanocomposite than that with pure g-C3N4, indicating the higher hydrogen evolution capability of this nanocomposite as the electro-catalyst.

Engineering high quality graphene superlattices via ion milled ultra-thin etching masks

Nanofabrication research pursues the miniaturization of patterned feature size. In the current state of the art, micron scale areas can be patterned with features down to ~30 nm pitch using electron beam lithography. Here, we demonstrate a nanofabrication technique which allows patterning periodic structures with a pitch down to 16 nm. It is based on focused ion beam milling of suspended membranes, with minimal proximity effects typical to standard electron beam lithography. The membranes are then transferred and used as hard etching masks. We benchmark our technique by electrostatically inducing a superlattice potential in graphene and observe bandstructure modification in electronic transport. Our technique opens the path towards the realization of very short period superlattices in 2D materials, but with the ability to control lattice symmetries and strength. This can pave the way for a versatile solid-state quantum simulator platform and the study of correlated electron phases.

Improving the thermoelectric performance of Cu-doped MoS2 film by band structure modification and microstructural regulation

Two-dimensional (2D) layered transitional metal dichalcogenides (TMDCs) semiconductors like MoS2 have been predicted to be promising thermoelectric (TE) materials due to their high carrier mobility. However, the TE performance of MoS2 film is severely hindered by the high in-plane thermal conductivity. In this work, based on reversing grain orientation reducing in-plane thermal conductivity, Cu doping is used to increase the carrier concentration for optimization of electrical properties. The Cu-doped MoS2 films have prepared by magnetron sputtering (MS) combined with post-sulfurization chemical vapor deposition (CVD). The composition, structure, thermal and electrical properties were systematically investigated by experiment and DFT calculation. Results indicate that Cu doping may slightly reduce the Seebeck coefficient of MoS2 film, but effectively narrow the band gap and improve the carrier concentration, thus improving the electrical performance. In addition, MS combined with CVD method can effectively increase the degree of grain misorientation, making the in-plane thermal conductivity of Cu-doped MoS2 film much lower. Combined with the above optimization, a promising maximum ZT value among TMDCs 0.137 at 425 K was obtained in Cu6-MoS2 film. This study provides a new strategy to synchronously regulate the thermal and electrical properties of TMDCs films to improve their thermoelectric performances.

Van der Waals lattice-induced colossal magnetoresistance in Cr2Ge2Te6 thin flakes

Recent discovery of two-dimensional (2D) magnets with van der Waals (vdW) gapped layered structure prospers the fundamental research of magnetism and advances the miniaturization of spintronics. Due to their unique lattice anisotropy, their band structure has the potential to be dramatically modulated by the spin configuration even in thin flakes, which is still unexplored. Here, we demonstrate the vdW lattice-induced spin modulation of band structure in thin flakes of vdW semiconductor Cr2Ge2Te6 (CGT) through the measurement of magnetoresistance (MR). The significant anisotropic lattice constructed by the interlayer vdW force and intralayer covalent bond induces anisotropic spin-orbit field, resulting in the spin orientation-dependent band splitting. Consequently, giant variation of resistance is induced between the magnetization aligned along in-plane and out-of-plane directions. Based on this, a colossal MR beyond 1000% was realized in lateral nonlocal devices with CGT acting as a magneto switch. Our finding provides a unique feature for the vdW magnets and would advance its applications in spintronics.

Effect of Bi3+ incoporation on up/downconversion luminescence and photocatalytic activity of Gd2O3

Gd2O3 has been selected as host material for optical studies due to its good chemical, photothermal, and photochemical stabilities. A series of (Gd1-xBix)2O3 (x = 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) samples were obtained via coprecipitation method. The samples were characterized by X-ray powder diffractometry (XRD), transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED), BET surface area method, UV-visible (UV–vis) absorption spectra and photoluminescence (PL) spectroscopy. The crystalline-to-amorphous phase transformation was found with higher concentration of Bi3+ incorporation. The dependence of Bi3+ concentration on the upconversion/downconversion luminescence intensity were investigated and the photocatalytic activity of the synthesized samples was also evaluated by the degradation of Rhodamine B (RhB) dye in aqueous solution under visible light irradiations. The samples (Gd0.5Bi0.5)2O3 exhibited better photocatalytic activity for RhB degradation than Gd2O3 and Bi2O3 due to phase transformation, modified band structures and higher surface area.

Improving thermoelectric properties of Cu2O powder via interface modification

We report thermoelectric properties of Cu2O powder through interface modification. Seebeck coefficient (SC) of Cu2O–Au composite powder was measured to be 900–1500 μV/K in the temperature range of 300–420 K, and power factor was ∼1.95 μW/m⋅K2 at 420 K, in contrast to raw Cu2O powder (0.86 μW/m⋅K2). The improved Seebeck coefficient and power factor relate very likely to the band structure modification by Au atoms, energy filtering and remote doping. Optic measurement and band structure calculated using density functional theory confirmed the band gap of Cu2O–Au composite powder (1.78eV) is slightly narrower than the raw Cu2O powder (1.85eV) and higher electronic density of states (DOS) in the conductance band of Cu2O–Au composite powder. Powder form thermoelectric devices (P-TE) easily prepared by pressing in a die without sintering might offer an alternative paradigm for mass fabrication with low cost. The high Seebeck coefficient and power factor of P-TE could find applications as power generator, temperature/touch sensors, transient thermoelectric coolers, etc.

Covalent coupling promoting charge transport of CdSeTe/UiO-66 for boosting photocatalytic CO2 reduction

Quantum dots (QDs) based heterojunction is a candidate for the photocatalytic CO2 reduction, owing to the large extinction coefficient and easy modification of band structures. However, the van der Waals interaction causes the large charge resistance and strong recombination centers between QDs and host materials, which makes the poor photocatalytic performance. Herein, a covalent bonded CdSeTe QDs and NH2-UiO-66 heterojunction (NUC-x) is constructed through an acylamino (-CONH-). The results indicate that the acylamino between NH2-UiO-66 and CdSeTe QDs can serve as the transfer channels for the photogenerated charges and stabilize the QDs. The optimized NUC-1200 achieved a CO generation rate of 228.68 µmol/g, which is 13 and 4 times higher than that of NH2-UiO-66 and CdSeTe QDs, respectively. This work provides a new avenue for efficient and stable photocatalysis of QDs.

Hot Carrier-Induced Nonlinear Photoluminescence in a Thin Indium Tin Oxide Layer Patterned by Ga Ion Beam Milling

This paper explores the nonlinear photoluminescence emitted by indium tin oxide (ITO) thin layers patterned by focused gallium ion beam milling. Using tightly focused near-infrared femtosecond pulsed laser excitation, a broad up-converted luminescence spectrum spanning the visible is detected. The intensity of the luminescence follows a nonmonotonous relationship with milling doses and can be related to the modification of the ITO electronic band structure by the implantation of Ga ions. The shape and the power dependence of the spectrum share strong similarities with those of nonlinear photoluminescence arising from metals. The results are consistent with a nonlinear process originating from the radiative decay of photogenerated hot carriers. The thermal coefficient relating the carrier temperature to the laser intensity is determined as a function of milling dose.

Coupling modification of Fermi level, band flattening and lattice defects to approach outstanding thermoelectric performance of ZnO films via tuning In and Ga incorporation

Band structure modification is found one of the state-of-the-art strategies to control electrical, thermal transport properties, and optimize performance of thermoelectric (TE) materials. Despite being a high-temperature n-type TE material, low-dimensional ZnO structures, specifically thin films normally possess a poor performance due to instability, low electrical, and high thermal conductivities. Herein, it is found that the trade-offs between TE parameters of ZnO films can be simultaneously abated by co-doping In and Ga. The dual incorporation of In and Ga enhances crystallinity, thermal stability, and TE properties up to 573 K. Tuning In and Ga contents not only optimizes carrier concentration associating with Fermi level modification, but also engineers lattice defects contributing to scattering transport mechanisms. Furthermore, the In3+ compensation at substitutional sites, specifically at high temperature, significantly increases density-of-state effective mass due to conduction band flattening. As a result, the coupling modification of Fermi level, band flattening, and lattice defects leads to maintaining electrical conductivity at a medium degree, increasing Seebeck coefficient, and decreasing thermal conductivity, respectively. The TE dimensionless figure of merit (ZT) and power factor (PF) of the films deposited from the In0.01Ga0.04Zn99.95O compound significantly boost to 0.2 and 745.2 µW/mK2 at 573 K, respectively. These values are 1328 and 536% higher than pristine ZnO film; 400 and 258% greater than Ga single-doped ZnO film, respectively. It proposes that our ZnO-based thin films approach the high ZT region (ZT ≥ 0.2) of advanced nanostructured ZnO-based bulks.

One-dimensional electronic states in a natural misfit structure

Misfit compounds are thermodynamically stable stacks of two-dimensional materials, forming a three-dimensional structure that remains incommensurate in one direction parallel to the layers. As a consequence, no true bonding is expected between the layers, with their interaction being dominated by charge transfer. In contrast to this well-established picture, we show that interlayer coupling can strongly influence the electronic properties of one type of layer in a misfit structure, in a similar way to the creation of modified band structures in an artificial moir\'e structure between two-dimensional materials. Using angle-resolved photoemission spectroscopy with a micron-scale light focus, we selectively probe the electronic properties of hexagonal ${\mathrm{NbSe}}_{2}$ and square BiSe layers that terminate the surface of the $({\mathrm{BiSe})}_{1+\ensuremath{\delta}}{\mathrm{NbSe}}_{2}$ misfit compound. We show that the band structure in the BiSe layers is strongly affected by the presence of the hexagonal ${\mathrm{NbSe}}_{2}$ layers, leading to quasi-one-dimensional electronic features. The electronic structure of the ${\mathrm{NbSe}}_{2}$ layers, on the other hand, is hardly influenced by the presence of the BiSe. Using density functional theory calculations of the unfolded band structures, we argue that the preferred modification of one type of band is mainly due to the atomic and orbital character of the states involved, opening a promising way to design electronic states that exploit the partially incommensurate character of the misfit compounds.