Carrier Electron Mobility Research Articles

Optoelectronic and Transport Properties of Layer-dependent Two-Dimensional Perovskite Cs3Bi2I9

The emergence of two-dimensional (2D) perovskite provides an ideal platform for designing and fabricating microelectronic and optoelectronic devices. Here, we present detailed ab initio calculations to comprehensively explore the layer-dependent optoelectronic and transport properties of the newly synthesized 2D perovskite Cs3Bi2I9. The calculations reveal that the indirect band gap of monolayer, bilayer, and trilayer Cs3Bi2I9 decreases from 2.41 to 2.35 eV. The charge carriers of few-layered perovskite Cs3Bi2I9 are dominated by electrons, and the highest electron and hole carrier mobilities are 140 and 62 cm²V⁻¹s⁻¹ along the b axis for bilayer Cs3Bi2I9. Exciton binding energies decrease from 2.48 to 1.19 eV with an increment of layer, and the calculated exciton level drops down into the valence band to generate potential exciton insulator. 2D Cs3Bi2I9 exhibits potential in the field of ultraviolet detection and photoluminescent devices due to large exciton energy and ultraviolet absorption.

Disentangling the Optoelectronic Behavior of Lead Iodide Governed by Two-Dimensional Electron Confinement.

We present a joint experimental and theoretical study for complete spectroscopic characterization and optoelectronic properties of lead iodide. Experimentally, we combine X-ray diffraction experiments to elucidate the structure with photoelectron spectroscopy to explore its electronic structure. Computationally, simulations are performed in the frame of density functional theory. We show that PbI2 presents a two-dimensional layered structure and exhibits a large transient photocurrent effect under visible light illumination, which are compatible with the surface photovoltage scenario. The transient photocurrent has an extremely long lifetime: when the sample is lightened with visible light, it shows very long relaxation times and, consequently, huge charge carrier diffusion lengths. We explain this anomalous behavior with the slow carrier mobility of holes and electrons caused by the 2D electron confinement in the layered material. Our results can be used as a simple model for understanding the optoelectronic properties of more complex 2D hybrid perovskites.

Two-Dimensional Janus XY (X/Y=P, As, Sb, Bi, X≠Y) material with excellent piezoelectricity and carrier mobility for Visible-Light water splitting

Janus materials, with unique physical and chemical properties, show promise in turning solar energy into clean hydrogen through photocatalytic water splitting. Herein, utilizing first-principles calculations, we have confirmed the stabilities of newly discovered two-dimensional XY materials (X/Y=P, As, Sb, Bi, X≠Y), unveiling their promising capabilities in photocatalytic water splitting. These Janus structures exhibit broad bandgaps, paired with remarkable electron carrier mobilities. Their out-of-plane piezoelectric coefficients (d31) varying from 0.10 pm/V to 0.31 pm/V, surpass those of many typical two-dimensional materials, highlighting their considerable potential in energy conversion applications. These materials stand out in water splitting due to their favorable band edges and electrostatic potential gradients, achieving solar-to-hydrogen (STH) efficiencies above 20 % and visible light absorption efficiencies over 10 %. Remarkably, Janus PBi showcases STH and visible light absorption efficiencies up to 29.01 % and 19.33 %, respectively. By using biaxial strain, Janus PAs and PSb can reach a peak STH efficiency of over 30 % and a top visible light absorption efficiency above 20 %. Janus AsSb (PBi) exhibits exceptionally high photocatalytic activity for the oxygen (hydrogen) evolution reaction. These attributes suggest that these Janus XY materials are promising contenders as photocatalytic catalysts for water-splitting applications.

A General Synthesis Method for Covalent Organic Framework and Inorganic 2D Materials Hybrids.

Two-dimensional (2D) inorganic/organic hybrids provide a versatile platform for diverse applications, including electronic, catalysis, and energy storage devices. The recent surge in 2D covalent organic frameworks (COFs) has introduced an organic counterpart for the development of advanced 2D organic/inorganic hybrids with improved electronic coupling, charge separation, and carrier mobility. However, existing synthesis methods have primarily focused on few-layered film structures, which limits scalability for practical applications. Herein, we present a general synthesis approach for a range of COF/inorganic 2D material hybrids, utilizing 2D inorganic materials as both catalysts and inorganic building blocks. By leveraging the intrinsic Lewis acid sites on the inorganic 2D materials such as hexagonal boron nitride (hBN) and transition metal dichalcogenides, COFs with diverse functional groups and topologies can grow on the surface of inorganic 2D materials. The controlled 2D morphology and excellent solution dispersibility of the resulting hybrids allow for easy processing into films through vacuum filtration. As proof of concept, hBN/COF films were employed as filters for Rhodamine 6G removal under flow-through conditions, achieving a removal rate exceeding 93%. The present work provides a simple and versatile synthesis method for the scalable fabrication of COF/inorganic 2D hybrids, offering exciting opportunities for practical applications such as water treatment and energy storage.

Improving the crystal quality and optoelectronic property of GaSb with Al doping

GaSb and Ga1-xAlxSb crystals were prepared with the vertical Bridgman (VB) method. The effects of aluminum atoms (Al) doping on the structure and photoelectric properties of GaSb crystal were investigated. The results show that the GaAlSb crystal maintains the zinc blende crystal structure of the GaSb crystal, and the crystal quality gradually improves with the increase of Al doping concentration, and the effective segregation coefficient keff of Al decreases from 3.69 to 2.83. Furthermore, Al doping significantly inhibits the formation of dislocations, of which density decreases from 3.634 × 103 cm−2 to 1.256 × 103 cm−2. As an element with the same equivalent electrons as Ga, Al doping makes GaSb crystal become into an n-type semiconductor with the free electrons as the main carriers, from a p-type semiconductor with the holes as the main carriers (8.867 × 1017 cm−3). And the carrier concentration, i.e., the free electrons concentration, increases with the doped Al concentration increase, up to the maximum of 1.929 × 1018 cm−3. And the mobility of electron carriers increases with the increase of Al concentration, up to 2.140 × 103 cm2/(V•s), far greater than that of the hole carriers 1.358 × 103 cm2/(V•s). And the resistivity decreases from 7.649 × 10−3 Ω•cm to 1.416 × 10−3 Ω•cm. Moreover, the infrared transmittance spectrum also shows a significant improvement of the crystal quality.

Interplay between cation composition and charge transport characteristics in GAxFAyMA1-x-yPbI3 halide perovskites

Halide perovskites (HPs) are a well-known class of mixed electronic and ionic conductors with diverse applications in optoelectronic devices. The simultaneous transport of ionic and electronic carriers has beneficial and detrimental effects depending on the intended applications. There is an extensive understanding of the charge transport characteristics in HPs since the phenomenon is of applied relevance. However, considering that several applications use compositions containing mixed cations, a deeper understanding of how the degree of substitution and the characteristics of the substituent cations affect the charge transport characteristics is needed. To this end, we experimentally studied the ionic conductivity (σion), current–voltage hysteresis (J–E hysteresis), mobility (μe) and density (ne) of electronic carriers, and bandgap energies (Eg) of up to 24 compositions of methylammonium lead iodide partially substituted with guanidinium and formamidinium. The results indicate that σion, J–E hysteresis, and μe decrease with the degree of substitution, with the J–E hysteresis being smaller the larger size of the substituent cation. At the same time, σion appears to be lower in compositions with equimolar substituents, in which the entropy of mixing is maximum. On the other hand, a slight increase in ne was observed with the substitution degree, showing highest values for FA+-rich compositions, where Eg is the lowest. The results advance the understanding of how it is possible to customize charge transport properties through the rational design of compositions in HPs.

Enhancement of thermoelectric performance in monolayer AlP3 via Ga and In doping: A first-principles study

Recently, two-dimensional (2D) triphosphides have attracted much attention in the thermoelectric field due to their semiconductor properties and excellent electronic transport properties. In our study, the performances of AlP3, Al0.5Ga0.5P3, and Al0.5In0.5P3 doped monolayers in the thermoelectric transport are studied. First, the dynamic structural stabilities of the three monolayers are verified through phonon dispersion curves. Moreover, all monolayers are indirect band-gap semiconductors, which can be used as thermoelectric materials. The results show that larger electronic relaxation time and carrier mobilities of doped monolayers lead to larger electronic transport parameters and higher dimensionless thermoelectric figure-of-merit (ZT). It is found that after doping via Ga and In, the thermoelectric performances of AlP3 are increased by 1.31–1.79 times. Our findings show that AlP3 and its doped monolayers hold significant promise as thermoelectric materials within the temperature span of 300–700 K and the doping mechanism could significantly improve the thermoelectric properties of 2D structures.

Flexible field-effect transistors with high-quality and uniform single-layer graphene for high mobility

In this work, a fully flexible graphene field-effect transistor with high carrier mobility is reported. Patterned high-quality and uniform single-layer graphene films are successfully realized by combining the selective growth on a patterned copper foil and the direct transfer method to minimize degradation factors. The selectively grown single-layer graphene is directly transferred to the target substrate through the deposition of poly-para-xylylene (Parylene) C. The quality of the graphene films is confirmed by Raman spectroscopy. The analysis reveals that the use of Parylene C as the substrate, gate dielectric, and encapsulation layer has the advantage of reducing the scattering by the optical phonons and charge puddles. The estimated residual carrier density is 1.72 × 1011 cm−2, and the intrinsic hole and electron carrier mobilities are found to be as high as 10 260 and 10 010 cm2 V−1 s−1, respectively. This study can pave the way for the development and mass production of high-performance and fully flexible graphene electronics.

Single-layer GaInS3: Water-splitting photocatalyst with high solar conversion efficiency and long carrier lifetime from first-principles investigation

Two-dimensional (2D) materials with a high solar to hydrogen (STH) conversion are in high demand due to the escalating energy crisis and environmental degradation. The STH efficiency is strongly correlated with the charge carrier separation, which can be improved by the inside electric dipole. Herein, the water-splitting capabilities of single-layer (SL) GaInS3 are explored by employing first-principles calculations. Theoretical outcomes reveal that SL GaInS3 exhibits a large intrinsic electric dipole and has a thermally stable structure at 300 K. SL GaInS3 displays a direct bandgap of 1.83 eV, as well as suitable band edges and abundant visible absorption (105 cm−1) for solar-driven water-splitting. The electronic structure and carrier mobility calculations demonstrate the high separation efficiency of charge carriers, which is strongly affirmed by the quite long carrier lifetime of 8.26 ns. The overpotential analysis of charge carriers suggests that the oxygen generation of SL GaInS3 can be accomplished without a cocatalyst, while hydrogen generation can be completed with the aid of cocatalysts. The STH efficiency of SL GaInS3 reaches up to 16.8%, apparently surpassing the commercial standard (10%). In brief, the high STH and long carrier lifetime promise the possible application of SL GaInS3 for solar water splitting. What's more, this work provides straight evidence that 2D photocatalysts with inside dipoles exhibit a long carrier lifetime.

Advancing intrinsic carrier mobility estimation in transition metal trichalcogenide monolayers using DFT-BTE

The Boltzmann transport equation (BTE) based intrinsic carrier mobility estimation significantly improves accuracy, which is crucial for assessing the performances of the materials within the devices. Herein, we explore the highly anisotropic, semiconducting 2D transition metal trichalcogenide monolayers (TMTC) MX3 (M = Ti, Zr, Hf and X = S, Se) for their transport properties. Remarkably, the electron carrier mobility obtained by combining BTE with density functional theory (DFT) in TiS3 monolayer has reached ∼1400 cm2/V.s. This finding stands in stark contrast to the electron mobility of ∼104 cm2/V·s obtained using a formalism built on the effective mass approximation. The marked disparity in mobility estimation underscores the crucial role played by the BTE in elevating precision. Alongside, a pronounced anisotropy in carrier mobility has been observed in these monolayers, particularly concerning lattice directions and electron-to-hole carrier mobility. Overall, this study seeks to fill out the voids and focuses on accurate estimation of high carrier mobility in TMTC monolayers using DFT-BTE.

How Biorecognition Affects the Electronic Properties of Reduced Graphene Oxide in Electrolyte‐Gated Transistor Immunosensors

AbstractAmbipolar electrolyte‐gated transistors (EGTs) based on reduced graphene oxide (rGO) have been demonstrated as ultra‐sensitive and highly specific immunosensors. However, the physics and chemistry ruling the device operation are still not fully unraveled. In this work, the aim is to elucidate the nature of the observed sensitivity of the device. Toward this aim, a physical–chemical model that, coupled with the experimental characterization of the rGO‐EGT, allows one to quantitatively correlate the biorecognition events at the gate electrode and the electronic properties of rGO‐EGT is proposed. The equilibrium of biorecognition occurring at the gate electrode is shown to determine the apparent charge neutrality point (CNP) of the rGO channel. The multiparametric analysis of the experimental transfer characteristics of rGO‐EGT reveals that the recognition events modulate the CNP voltage, the excess carrier density Δn, and the quantum capacitance of rGO. This analysis also explains why hole and electron carrier mobilities, interfacial capacitance, the curvature of the transfer curve, and the transconductances are insensitive to the target concentration. The understanding of the mechanisms underlying the transistor transduction of the biorecognition events is key for the interpretation of the response of the rGO‐EGT immunosensors and to guide the design of novel and more sensitive devices.

Carrier mobility and optical properties of a type-II GaSe/ZnS heterostructure as a photocatalyst: a first-principles study.

In this paper, a new GaSe/ZnS van der Waals heterostructure (vdWH) was constructed and a systematic analysis of the electronic structure, interfacial properties, and transport and photocatalytic capacity of the GaSe/ZnS vdWH was performed by using first-principles calculations. It was found that the heterostructure exhibited excellent photocatalytic performance for water splitting. The direct band gap of the heterostructure calculated using the hybrid HSE06 functional was 2.19 eV, which had a good visible light absorption ability. The electronic structure of the type-II band arrangement effectively reduced the recombination of electron-hole pairs. The heterostructure also showed excellent transport ability, and the carrier mobility of electrons and holes along different directions was greatly improved. Additionally, as the electric field strength increased, the band gap width of the GaSe/ZnS vdWH narrowed and the heterostructure characteristics transitioned from semiconductor to metal properties, which were attributed to the appearance of near-free electronic (NFE) states induced by the strong electric field. Meanwhile, the optical absorption capacity of the heterostructure was greatly improved compared to the ZnS monolayer, reaching 1.44 × 105 cm-1 at an incident photon energy of 8.65 eV. Therefore, the GaSe/ZnS vdWH was proved to be an excellent photocatalytic material for water splitting in the present study.

Organic cations in halide perovskite solid solutions: exploring beyond size effects.

Halide perovskites are a class of materials of consolidated optoelectronic and electrochemical applications, reaching efficiencies compared to established materials in respective fields. In this scenario, the design and understanding of composition-structure-property relations is imperative. In solid solutions containing mixed cations, some direct relations between the sizes of the substituents and the properties of perovskites are generally observed. However, in several cases, these relations are not observed, implying that other characteristics of these cations play a major role. Despite its importance, this understanding has not been comprehensively deepened. To address this issue, we synthesized and characterized the structure, electrical behavior, and stability of methylammonium lead iodide-based perovskites with equal amounts of the substituents guanidinium, ethylammonium, and acetamidinium. These three large organic cations have essentially equal sizes but other remarkably different characteristics, such as the number of N-H bonds, intrinsic dipole moment, and order of C-N bonds. Herein, we show that these cations have dramatically different effects over important fundamental and applied properties of resulting perovskites, including the orthorhombic-to-tetragonal and tetragonal-to-cubic phase transitions, microstructural development, ionic conductivity, I-V hysteresis, electronic carrier mobility, and stability against light-induced degradation. These effects are correlated with the characteristics of the large substituent cations and help pave the way for a better rational chemical design of halide perovskites.

Carrier transport in bulk and two-dimensional Zn2(V,Nb,Ta)N3 ternary nitrides.

Density functional theory-based simulations are applied to study the electronic structures, carrier masses, carrier mobility and carrier relaxation times in bulk and two-dimensional (2D) Zn2(V,Nb,Ta)N3 ternary nitrides. Bulk Zn2(V,Nb,Ta)N3 possess moderate band gap sizes of 2.17 eV, 3.11 eV, and 3.40 eV, respectively. Two-dimensional Zn2(V,Nb,Ta)N3 have slightly higher band gap sizes of 2.77 eV, 3.33 eV, and 3.23 eV, respectively. Carrier mass, carrier mobility and carrier relaxation time are found to be anisotropic in all the studied structures. Bulk and 2D samples show an order of magnitude higher electron mobility compared to hole mobility. The highest electron mobility in bulk Zn2NbN3 and Zn2TaN3 is about ∼103 cm2 V-1 s-1. Importantly, for 2D Zn2NbN3, an abnormally high electron mobility of 1.67 × 104 cm2 V-1 s-1 is observed, which is not inferior to the highest known electron mobility values in 2D materials. Such a high electron mobility in 2D Zn2NbN3 can be attributed to a strong delocalization of the conduction band minimum, which is responsible for electron transport. Therefore, this work opens up new materials for high performance nanodevices, such as tandem solar cells and field-effect transistors. This study also provides deep physical insights into the nature of carrier transport mechanisms in bulk and 2D Zn2(V,Nb,Ta)N3 ternary nitrides.

Revealing large room-temperature Nernst coefficients in 2D materials by first-principles modeling.

Two-dimensional (2D) materials have attracted significant attention owing to their distinctive electronic, thermal, and mechanical characteristics. Recent advancements in both theoretical understanding and experimental methods have greatly contributed to the understanding of thermoelectric properties in 2D materials. However, thermomagnetic properties of 2D materials have not yet received the same amount of attention. In this work, we select promising 2D materials guided by the physics of the Nernst effect and present a thorough first-principles study of their electronic structures, carrier mobilities, and Nernst coefficients as a function of carrier concentration. Specifically, we reveal that trilayer graphene with an ABA stacking exhibits an exceptionally large Nernst coefficient of 112 μV (KT)-1 at room temperature. We further demonstrate that monolayer graphene, ABC-stacked trilayer graphene, and trilayer phosphorene (AAA stacking) have large Nernst coefficients at room temperature. This study establishes an ab initio framework for the quantitative study of the thermomagnetic effects in 2D materials and demonstrates high fidelity with previous experimental data.

The effects of MS2 (M = Mo or W) substrates on electronic properties under electric fields in germanene-based field-effect transistors

Germanene has attracted significant attention due to its novel electronic properties and strong spin-coupling effect. However, the tiny band gap of the germanene dramatically limits its application in field-effect transistors (FETs). Inspired by the utilization of the substrates and electric fields to adjust the band gaps of two-dimensional materials, we investigated the fundamental mechanism of electric fields on the atomic structures and electronic properties of germanene supported by MS2 (M = Mo or W) substrates through first-principles calculation. The results show that the substrates can induce a symmetry breaking in the germanene sublattice via van der Waals interaction, leading to a sizable band gap at the Dirac point. In addition, the band gaps of the germanene/MS2 heterostructures can be effectively modulated by applying an external electric field. Under suitable electric fields, the considerable band gap values of CMo germanene/MoS2 and TGeL-W germanene/WS2 configurations can open the maximum band gaps with 263 and 247 meV, which satisfy the requirements of FETs at room temperature. Meanwhile, the evolutions of charge transfers under electric fields were explored to illustrate how electric fields and substrates promote the electronic properties of germanene. More interestingly, a Schottky–Ohmic transition can occur when a specific electric field is imposed on the germanene/MS2 heterostructures. Note that the hole and electron carrier mobilities of germanene/MS2 heterostructures are still significantly preserved, showing some superior electronic performances than some heterostructures. The results provide a critical theoretical guide for improving the electronic properties of germanene, and demonstrate the designed germanene/MS2 heterostructures with the tunable band gaps and higher carrier mobilities as germanene-based FETs.

Unveiling the potential of two-dimensional Janus ScXAlY (X/Y=P, As) materials and heterostructures in visible-light-driven photocatalytic water splitting

Two-dimensional (2D) material with Janus-induced built-in electric field, has become a research focus arising from their characteristic physical and chemical properties, especially in photocatalytic water splitting applications. Herein, we introduce a novel family of 2D Janus materials, named ScXAlY (X/Y = P, As), and prove these Janus materials and their heterostructures as promising water splitting photocatalytic candidate materials, leveraging first-principles calculations. All Janus structures and heterostructures exhibit strong dynamical, thermal, and mechanical stabilities. These 2D Janus ScXAlY materials possess wide band gaps, excellent electronic carrier mobilities, and appropriate band edges to facilitate overall water splitting, and moreover can attain Janus-induced built-in electric field effect. Interestingly, the heterostructures (ScXAlY/ScXAlY) surpass the individual materials in performance, demonstrating enhanced solar-to-hydrogen (STH) efficiencies (>30 %) and increscent electrostatic potential differences (>0.85 eV). Furthermore, they show comprehensive photoabsorption across the visible light spectrum, with peak absorptions more than doubling those of the single materials. These heterostructures also display effective spatial separation of electrons and holes which can further boosting their photocatalytic capabilities. Our findings underscore the potential of Janus ScXAlY materials and their heterostructures, in facilitating visible-light-driven photocatalytic water splitting, showcasing a promising avenue for advancement in this burgeoning field.

Tunable Electronic Properties, Carrier Mobility, and Contact Characteristics in Type-II BSe/Sc2CF2 Heterostructures toward Next-Generation Optoelectronic Devices.

Conducting heterostructures have emerged as a promising strategy to enhance physical properties and unlock the potential application of such materials. Herein, we conduct and investigate the electronic and transport properties of the BSe/Sc2CF2 heterostructure using first-principles calculations. The BSe/Sc2CF2 heterostructure is structurally and thermodynamically stable, indicating that it can be feasible for further experiments. The BSe/Sc2CF2 heterostructure exhibits a semiconducting behavior with an indirect band gap and possesses type-II band alignment. This unique alignment promotes efficient charge separation, making it highly promising for device applications, including solar cells and photodetectors. Furthermore, type-II band alignment in the BSe/Sc2CF2 heterostructure leads to a reduced band gap compared to the individual BSe and Sc2CF2 monolayers, leading to enhanced charge carrier mobility and light absorption. Additionally, the generation of the BSe/Sc2CF2 heterostructure enhances the transport properties of the BSe and Sc2CF2 monolayers. The electric fields and strains can modify the electronic properties, thus expanding the potential application possibilities. Both the electric fields and strains can tune the band gap and lead to the type-II to type-I conversion in the BSe/Sc2CF2 heterostructure. These findings shed light on the versatile nature of the BSe/Sc2CF2 heterostructure and its potential for advanced nanoelectronic and optoelectronic devices.

Strain engineering on electronic structure, effective mass and charge carrier mobility in monolayer YBr3

In recent years, two-dimensional materials have significant prospects for applications in nanoelectronic devices due to their unique physical properties. In this paper, the strain effect on the electronic structure, effective mass, and charge carrier mobility of monolayer yttrium bromide (YBr3) is systematically investigated using first-principles calculation based on density functional theory. It is found that the monolayer YBr3 undergoes energy band gap reduction under the increasing compressive strain. The effective mass and charge carrier mobility can be effectively tuned by the applied compressive strain. Under the uniaxial compressive strain along the zigzag direction, the hole effective mass in the zigzag direction (m ao1_h) can decrease from 1.64 m 0 to 0.45 m 0. In addition, when the uniaxial compressive strain is applied, the electron and hole mobility can up to ∼103 cm2 V−1 s−1. The present investigations emphasize that monolayer YBr3 is expected to be a candidate material for the preparation of new high-performance nanoelectronic devices by strain engineering.

CEEM: a Chemically Explainable Deep Learning Platform for Identifying Compounds with Low Effective Mass.

The semiconductor industry occupies a crucial position in the fields of integrated circuits, energy, and communication systems. Effective mass (mE ), which is closely related to electron transition, thermal excitation, and carrier mobility, is a key performance indicator of semiconductor. However, the highly neglected mE is onerous to measure experimentally, which seriously hinders the evaluation of semiconductor properties and the understanding of the carrier migration mechanisms. Here, a chemically explainable effective mass predictive platform (CEEM) is constructed by deep learning, to identify n-type and p-type semiconductors with low mE . Based on the graph network, a versatile explainable network is innovatively designed that enables CEEM to efficiently predict the mE of any structure, with the area under the curve of 0.904 for n-type semiconductors and 0.896 for p-type semiconductors, and derive the most relevant chemical factors. Using CEEM, the currently largest mE database is built that contains 126335 entries and screens out 466 semiconductors with low mE for transparent conductive materials, photovoltaic materials, and water-splitting materials. Moreover, a user-friendly and interactive CEEM web is provided that supports query, prediction, and explanation of mE . CEEM's high efficiency, accuracy, flexibility, and explainability open up new avenues for the discovery and design of high-performance semiconductors.