Anisotropic Carrier Mobility Research Articles

Theoretical studies of electronic transport in monolayer and bilayer phosphorene: A critical overview

Recent ab initio theoretical calculations of the electrical performance of several two-dimensional materials predict a low-field carrier mobility that spans several orders of magnitude (from $26\phantom{\rule{0.16em}{0ex}}000$ to 35 ${\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, for example, for the hole mobility in monolayer phosphorene) depending on the physical approximations used. Given this state of uncertainty, we review critically the physical models employed, considering phosphorene, a group-V material, as a specific example. We argue that the use of the most accurate models results in a calculated performance that is at the disappointing lower end of the predicted range. We also employ first-principles methods to study high-field transport characteristics in monolayer and bilayer phosphorene. For thin multilayer phosphorene we confirm the most disappointing results, with a strongly anisotropic carrier mobility that does not exceed $\ensuremath{\sim}30$ ${\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 300 K for electrons along the armchair direction. We also discuss the dependence of low-field carrier mobility on the thickness of multilayer phosphorene.

γ-Graphyne analogues based on As and Sb elements

Identifying graphyne-based materials is being of great significance and urgently needed for the applications in next-generation devices and photocatalysis. In this work, we proposed a novel two-dimensional γ-graphyne derivative using As and Sb elements (γ-As and γ-Sb) from first-principles calculations. The calculated cohesive energy and phonon spectra provide convincing evidences for their stabilities and experiential feasibility. γ-As and γ-Sb monolayers possess direct band gaps of 1.83 and 1.36 eV, respectively. Strain effect can achieve efficient band gap engineering for these two novel 2D materials, which is highly desired for practical applications. Importantly, the anisotropy of carrier mobility up to 1517.24 cm2V-1s-1. also identified. Furthermore, we demonstrate that both band edges and band gaps of γ-As monolayer fulfill the requirements of the reduction and oxidation levels in water splitting. Therefore, our predictions not only introduce new vitality into the relevant researches of graphyne, enriching available candidate materials in this field, but also highlight the potential of γ-As and γ-Sb monolayers as appealing materials for water splitting and nano devices.

A New Anisotropic Dirac Cone Material: A B2S Honeycomb Monolayer.

Different from the isotropic Dirac cones existing in other two-dimensional (2D) materials, anisotropic Dirac cones have the merit of anisotropic carrier mobility for applications in direction-dependent quantum devices. Motivated by the recent experimental finding of an anisotropic Dirac cone in borophene, here we report a new 2D anisotropic Dirac cone material, B2S monolayer, identified by using a global structure search method and first-principles calculation combined with a tight-binding model. The B2S monolayer is found to be stable mechanically, thermally, and dynamically and exhibits an anisotropic Dirac cone exactly at the Fermi level, showing a Fermi velocity of 106 m/s in the same order of magnitude as that of graphene. Moreover, B2S monolayer is the first anisotropy Dirac cone material with a pristine honeycomb structure stabilized by S in free-standing conditions where each atom has four valence electrons on average being isoelectronic to C. This study would expand the Dirac cone material family with new features.

Anisotropic carrier mobility in single- and bi-layer C3N sheets

Based on the density functional theory combined with the Boltzmann transport equation with relaxation time approximation, we investigate the electronic structure and predict the carrier mobility of single- and bi-layer newly fabricated 2D carbon nitrides C3N. Although C3N sheets possess graphene-like planar hexagonal structure, the calculated carrier mobility is remarkably anisotropic, which is found mainly induced by the anisotropic effective masses and deformation potential constants. Importantly, we find that both the electron and hole mobilities are considerable high, for example, the hole mobility along the armchair direction of single-layer C3N sheets can arrive as high as 1.08×104 cm2 V−1 s−1, greatly larger than that of C2N-h2D and many other typical 2D materials. Owing to the high and anisotropic carrier mobility and appropriate band gap, single- and bi-layer semiconducting C3N sheets may have great potential applications in high performance electronic and optoelectronic devices.

Monolayer CS as a metal-free photocatalyst with high carrier mobility and tunable band structure: a first-principles study

Producing hydrogen fuel using suitable photocatalysts from water splitting is a feasible method to harvest solar energy. A desired photocatalyst is expected to have suitable band gap, moderate band edge position, and high carrier mobility. By employing first-principles calculations, we explore a α-CS monolayer as a metal-free efficient photocatalyst. The α-CS monolayer shows good energetic, dynamic, and thermal stabilities and is insoluble in water, suggesting its experimental practicability. Monolayer and bilayer α-CS present not only appropriate band gaps for visible and ultraviolet light absorption but also moderate band alignments with water redox potentials in pH neutral water. Remarkably, the α-CS monolayer exhibits high (up to 8453.19 cm2 V−1s−1 for hole) and anisotropic carrier mobility, which is favorable to the migration and separation of photogenerated carriers. In addition, monolayer α-CS experiences an interesting semiconductor-metal transition by applying uniaxial strain and external electric field. Moreover, α-CS under certain strain and electric field is still dynamically stable with the absence of imaginary frequencies. Furthermore, we demonstrate that the graphite (0 0 1) surface is a potential substrate for the α-CS growth with the intrinsic properties of α-CS maintaining. Therefore, our results could pave the way for the application of α-CS as a promising photocatalyst.

Theoretical insight into the carrier mobility anisotropy of organic-inorganic perovskite CH3NH3PbI3

High mobility, which is highly relevant to crystal structures, is one of the predominant superiorities of organic–inorganic halide perovskites. However, the anisotropy of carrier mobility for this photoelectric material on different crystal planes is still unclear. Based on Marcus theory and Density Functional Theory, we investigated the anisotropy of carrier mobility by calculating the intramolecular vibration (namely, internal recombination energy λ) and intermolecular electronic coupling integral V along a representative crystal plane. Results show that the electrons and holes exhibit consistency in the transport orientations that are parallel to the (010), (101), and (110) crystal planes. However, inconsistency was observed in those parallel to the (111) and (001) crystal planes (with an angle of approximately 60°) between the electron and hole transport directions This condition is unfavorable to the balancing of the transport and collection of photoinduced carriers. Our work reveals the theoretical significance of control-oriented growth of perovskites.

Two dimensional allotropes of arsenene with a wide range of high and anisotropic carrier mobility.

Considering the rapid development of experimental techniques for fabricating 2D materials in recent years, various monolayers are expected to be experimentally realized in the near future. Motivated by the recent research activities focused on the honeycomb arsenene monolayers, the stability and carrier mobility of non-honeycomb and porous allotropic arsenene are determined using first principles calculations. In addition to five honeycomb structures of arsenene, a total of eight other structures are considered in this study. An extensive analysis comprising energetics, phonon spectra and mechanical properties confirms that these structures are energetically and dynamically stable. All these structures are semiconductors with a broad range of band gaps varying from ∼1 eV to ∼2.5 eV. Significantly, these monolayer allotropes possess anisotropic carrier mobilities as high as several hundred cm2 V-1 s-1 which is comparable with well-known 2D materials such as black phosphorene and monolayer MoS2. Combining such broad band gaps and superior carrier mobilities, these monolayer allotropes can be promising candidates for the superior performance of the next generation nanoscale devices. We further explore these monolayer allotropes for photocatalytic water splitting and find that arsenene monolayers have potential for usage in visible light driven photocatalytic water splitting.

Nitrophosphorene: A 2D Semiconductor with Both Large Direct Gap and Superior Mobility

A new two-dimensional phosphorus nitride monolayer (P21/c-PN) with distinct structural and electronic properties is predicted based on first-principle calculations. Unlike pristine single-atom group V monolayers such as nitrogene, phosphorene, arsenene, and antimonene, P21/c-PN has an intrinsic direct band gap of 2.77 eV that is very robust against the strains. Strikingly, P21/c-PN shows excellent anisotropic carrier mobility up to 290 829.81 cm2 V–1 s–1 along the a direction, which is about 18 times that in monolayer black phosphorus. This put P21/c-PN way above the general relation that carrier mobility is inversely proportional to bandgap, making it a very unique two-dimensional material for nanoelectronics devices.

A theoretical approach for simulations of anisotropic charge carrier mobility in organic single crystal semiconductors

Abstract Charge carrier mobility is important for organic semiconductor materials and mainly determines their device performance. Thereby how to improve carrier mobility lies at the heart of the development of organic electronics. Theoretical predictions and simulations can provide guidelines towards the possible realization of high mobility and the design of highly functional semiconductor materials and thus can help to achieve further discoveries in the field. In this paper, we review a recently proposed theoretical method (an effective one-dimensional diffusion equation model) which presents the first analytical expression for angular resolution anisotropic mobility of organic single crystal semiconductors. The method encompasses the hopping mechanism, Marcus-Hush theory and first-principles calculations and is suitable to characterizing the anisotropic transport behaviors in organic single crystal semiconductors as well as to studying the property-structure relationship in semiconductor materials. Illustration of applications of the method demonstrated its capabilities in description and understanding of the transport of charges, correct prediction of angular resolution anisotropic mobility, and assist in the design of n-type and p-type organic electronic materials.

Proximity Effect Induced Spin Injection in Phosphorene on Magnetic Insulator

Black phosphorus is a promising candidate for future nanoelectronics with a moderate electronic band gap and a high carrier mobility. Introducing the magnetism into black phosphorus will widely expand its application scope and may present a bright prospect in spintronic nanodevices. Here, we report our first-principles calculations of spin-polarized electronic structure of monolayer black phosphorus (phosphorene) adsorbed on a magnetic europium oxide (EuO) substrate. Effective spin injection into the phosphorene is realized by means of interaction with the nearby EuO(111) surface, i.e., proximity effect, which results in spin-polarized electrons in the 3p orbitals of phosphorene, with the spin polarization at Fermi level beyond 30%, together with an exchange-splitting energy of ∼0.184 eV for conduction-band minimum of the adsorbed phosphorene corresponding to an energy region where only one spin channel is conductive. The energy region of these exchange-splitting and spin-polarized band gaps of the adsorbed phosphorene can be effectively modulated by in-plane strain. Intrinsically high and anisotropic carrier mobilities at the conduction-band minimum of the phosphorene also become spin-polarized mainly due to spin polarization of deformation potentials and are not depressed significantly after the adsorption. These extraordinary properties would endow black phosphorus with great potentials in the future spintronic nanodevices.

Theoretical insight into the carrier mobility anisotropy of hole transport material Spiro-OMeTAD

All-solid-state organic-inorganic halide perovskite solar cells (PSCs) have attracted wide attention due to the rapid progress of power conversion efficiency in recent years. Hole transport material (HTM) in PSCs plays the role of extracting and transporting photo-excited holes. Anisotropy of carrier mobility is one important property for semiconductors, however, which still remains unclear for the dominant HTM spiro-OMeTAD used in PSCs. Based on Density Functional Theory (DFT) and Marcus theory, we for the first time conducted investigations on the anisotropy of carrier mobility along representative crystal planes of spiro-OMeTAD by recombination energy λ and electronic coupling integral V. Results indicate that the holes and electrons show transport orientations consistency parallel to the (010), (101) and (111) crystal planes while inconsistency was found parallel to (100), (110), (011) and (001) crystal planes (with an angle ranged from 40° to 70° between the hole and electron transport directions). Our work embodies the theoretical significance of controllable and oriented fabrication of HTM in PSCs.

Two-dimensional germanium monochalcogenides for photocatalytic water splitting with high carrier mobility

Highly efficient utilization of solar energy to split water into hydrogen and oxygen is regarding as a promising strategy to deal with the future energy crisis and environmental problems. To explore highly efficient and low-cost photocatalysts is highly desired. Herein, phosphorene-like germanium monochalcogenides (GeS and GeSe monolayers) are proposed here as efficient photocatalysts for water splitting. After confirming their stabilities, we observe that GeS exhibits an indirect band gap of 2.29eV while GeSe reveals a direct band gap of 1.59eV by HSE hybrid functional. Remarkably, high and directionally anisotropic carrier mobilities (2430.50cm2V−1s−1 for GeS and 4032.64cm2V−1s−1 for GeSe) are quantitatively investigated by using deformation potential theory. In addition, GeS and GeSe monolayers exhibit a good separation of electrons and holes, which effectively reduces the photocatalytic activity with high efficiency. Upon the application of strain, the band structure can be modulated from semiconductor to metal and a direct-indirect bandgap transition is observed. Most intriguingly, the band gaps and band edge alignments at certain pH value can be effectively tuned to meet the requirement of the redox potential in water splitting. Finally, the adsorption and decomposition of water molecules on the surface of 2D GeS and the subsequent formation of hydrogen were explored, which unravels the mechanism of photocatalytic hydrogen production on 2D GeS. Our findings will be valuable for facilitating the exploration and application of GeS and GeSe for photocatalytic water splitting.

Mid-infrared ultrafast carrier dynamics in thin film black phosphorus

Black phosphorus is emerging as a promising semiconductor for electronic and optoelectronic applications. To study fundamental carrier properties, we performed ultrafast femtosecond pump-probe spectroscopy on thin film black phosphorus mechanically exfoliated on a glass substrate. Carriers (electrons and holes) were excited to high energy levels and the process of carrier relaxation through phonon emission and recombination was probed. We used a wide range of probing wavelengths up to and across the band gap to study the evolution of the relaxation dynamics at different energy levels. Our experiments revealed a plethora of important physical phenomena. The fast relaxation time constants, associated with carrier-phonon scattering, steadily increase as the energy of the probe beam approaches the band gap energy, which was determined to be 0.31 eV, and the carrier recombination rate was obtained when the probe wavelength was tuned to match the band gap energy. The carrier-phonon scattering rates were found to be similar along the armchair and zigzag directions, therefore, the anisotropic carrier mobility reported in literature is mainly due to the difference in effective mass of carriers along different directions. The ultrafast spectroscopy data further revealed the oxidation induced surface charges. Our results highlight the importance of using the spectroscopy technique, in this case, in the mid-IR range, to uncover useful physical processes.

Anisotropic charge transport properties in boron sub phthalocyanine chloride thin films

Anisotropic charge transport properties in organic semiconducting thin films of Boron Sub Phthalocyanine Chloride (Sub-Pc) have been studied by admittance spectroscopy applying a bias voltage parallel and perpendicular to the substrate. Sub-Pc long ordered structures confined in different directions induce anisotropic carrier mobility in films. The film with a thickness of 11 nm exhibited carrier mobilities three orders higher along the substrate than perpendicular to it. Furthermore, AFM, Kelvin Probe, and UV-Vis spectroscopic studies corroborated the established mobility-structure relationship.

Anisotropic carrier mobility in buckled two-dimensional GaN.

Developing nanoelectronic engineering requires two-dimensional (2d) materials with both usable carrier mobility and proper large band-gap. In this study, we present a detailed theoretical investigation of the intrinsic carrier mobilities of buckled 2d GaN. This buckled 2d GaN is accessed by hydrofluorination (FGaNH) and hydrogenation (HGaNH). We predict that the anisotropic carrier mobilities of buckled 2d GaN can exceed those of 2d MoS2 and can be altered by an alterable surface chemical bond (convert from a Ga-F-Ga bond of FGaNH to a Ga-H bond of HGaNH). Moreover, converting FGaNH to HGaNH can significantly suppress hole mobility (even close to zero) and result in a transition from a p-type-like semiconductor (FGaNH) to an n-type-like semiconductor (HGaNH). These features make buckled 2d GaN a promising candidate for application in future conductivity-adjustable electronics.

Two-dimensional GaGeTe film: a promising graphene-like material with tunable band structure and high carrier mobility

Two-dimensional GaGeTe film is promising for the photovoltaic emitting devices due to its tunable band structure, accompanied with anisotropic carrier mobility.

Anisotropic carrier mobility in two-dimensional materials with tilted Dirac cones: theory and application.

We have theoretically investigated the intrinsic carrier mobility in semimetals with tilted Dirac cones under both longitudinal and transverse acoustic phonon scattering. An analytical formula for the carrier mobility was obtained. It shows that tilting significantly reduces the mobility. The theory was then applied to 8B-Pmmn borophene and borophane (fully hydrogenated borophene), both of which have tilted Dirac cones. The predicted carrier mobilities in 8B-Pmmn borophene at room temperature are 14.8 × 105 and 28.4 × 105 cm2 V-1 s-1 along the x and y directions, respectively, both of which are higher than that in graphene. For borophane, despite its superhigh Fermi velocity, the carrier mobility is lower than that in 8B-Pmmn owing to its smaller elastic constant under shear strain.

Direct evaluation of anisotropic carrier mobility in uniaxially aligned polymer semiconductor film by time-resolved microscopic optical second-harmonic generation measurement

Mobility anisotropy in uniaxially-aligned fluorene co-polymer thin film was directly observed by using time-resolved microscopic optical second-harmonic generation (TRM-SHG) imaging. Main-chain orientation of fluorene co-polymer was determined by polarized absorption measurement, and the mobilities in the direction parallel and perpendicular to the main-chain were respectively estimated as cm2 Vs−1 and cm2 Vs−1 from the visualized carrier motion starting from a round-shape electrode. These results indicate that the mobility anisotropy of this sample was 4.0. Activation energy for each direction was also evaluated by the temperature dependence measurement as 117 and 94 meV, respectively. The TRM-SHG method enables us to estimate mobility and activation energy of the oriented polymer film in all directions at once.

The thermal and electrical properties of the promising semiconductor MXene Hf2CO2.

With the growing interest in low dimensional materials, MXenes have also attracted considerable attention recently. In this work, the thermal and electrical properties of oxygen-functionalized M2CO2 (M = Ti, Zr, Hf) MXenes are investigated using first-principles calculations. Hf2CO2 is determined to exhibit a thermal conductivity better than MoS2 and phosphorene. The room-temperature thermal conductivity along the armchair direction is determined to be 86.25~131.2 Wm−1 K−1 with a flake length of 5~100 μm. The room temperature thermal expansion coefficient of Hf2CO2 is 6.094 × 10−6 K−1, which is lower than that of most metals. Moreover, Hf2CO2 is determined to be a semiconductor with a band gap of 1.657 eV and to have high and anisotropic carrier mobility. At room temperature, the Hf2CO2 hole mobility in the armchair direction (in the zigzag direction) is determined to be as high as 13.5 × 103 cm2V−1s−1 (17.6 × 103 cm2V−1s−1). Thus, broader utilization of Hf2CO2, such as the material for nanoelectronics, is likely. The corresponding thermal and electrical properties of Ti2CO2 and Zr2CO2 are also provided. Notably, Ti2CO2 presents relatively lower thermal conductivity but much higher carrier mobility than Hf2CO2. According to the present results, the design and application of MXene based devices are expected to be promising.

Evaluation of anisotropic charge carrier mobility of perylene single crystals by time-of-flight method

The charge carrier mobilities along the vertical and lateral directions of perylene platelet single crystals were measured by the time-of-flight (TOF) method. In the lateral directional measurement, the entire region between electrodes was irradiated to obtain measurable signals. The transient photocurrent was different from the conventional TOF measurements; hence, we developed an analytic method for lateral directional measurement. The electron mobilities along the thickness and lateral directions were 0.33 and 2.0 cm2·V−1·s−1 and the hole mobilities were 0.12 and 0.6 cm2·V−1·s−1, respectively.