Intrinsic Mobility Research Articles

Thermal conductivity of molybdenum disulfide nanotube from molecular dynamics simulations

Single layer molybdenum disulfide (SLMoS2), a semiconductor possesses intrinsic bandgap and high electron mobility, has attracted great attention due to its unique electronic, optical, mechanical and thermal properties. Although thermal conductivity of SLMoS2 has been widely investigated recently, less studies focus on molybdenum disulfide nanotube (MoS2NT). Here, the comprehensive temperature, size and strain effect on thermal conductivity of MoS2NT are investigated. Thermal conductivity is obtained as 16 Wm−1 K−1 at room temperature, and it has a ∼T−1 dependence on temperature from 200 to 400 K and a ∼Lβ dependence on length from 10 to 320 nm. Interestingly, a chirality-dependent strain effect is identified in thermal conductivity of zigzag nanotube, in which the phonon group velocity can be significantly reduced by strain. This work not only provides feasible strategies to modulate the thermal conductivity of MoS2NT, but also offers useful insights into the fundamental mechanisms that govern the thermal conductivity, which can be used for the thermal management of low dimensional materials in optical, electronic and thermoelectrical devices.

Virus Size Analysis by Gas-Phase Mobility Measurements: Resolution Limits.

Electrospraying (ES) dissolved viral particles, followed by charge reduction and size analysis with a differential mobility analyzer (DMA), offers a flexible size-analysis tool for small particles in solution. The technique relies on pioneering work by Kaufman and colleagues, commercialized by TSI, and often referred to as GEMMA. However, viral studies with TSI's GEMMA have suffered from limited resolving power, possibly because of imperfections in either the instrument (DMA or charge reduction) or the sample solution preparation. Here, we explore the limits of the resolution achievable by GEMMA, taking advantage of (i) cleaner charge reduction methods and (ii) DMAs of higher resolving power. Analysis of the literature provides indications that mobility peak widths (fwhm) of 2% or less may be achieved by combining careful sample preparation with improved instrumentation. Working with purified PP7 bacteriophage particles small enough to be classifiable by existing high-resolution DMAs, we confirm that fairly narrow viral mobility peaks may be obtained (relative full width at half-maximum fwhm <5%). Comparison of spectra of a given apian virus sample obtained with TSI's GEMMA and our improved instrumentation confirms that one critical limitation is the DMA. This is further verified by narrow peaks from murine parvovirus, norovirus, and encephalomyelitis virus samples, obtained in our improved GEMMA with little sample preparation, directly from infected cell cultures. Classification of purified large (60 nm) coliphage PR772 particles leads to broad peaks, due to both viral degradation and limited intrinsic resolution of the DMAs used to cover the range of such large particles. We conclude that improved DMAs suitable for high-resolution analysis of particles larger than 30 nm need to be developed to determine the intrinsic mobility width of viral particles.

Route to High Hole Mobility in GaN via Reversal of Crystal-Field Splitting.

A fundamental obstacle toward the realization of GaN p-channel transistors is its low hole mobility. Here we investigate the intrinsic phonon-limited mobility of electrons and holes in wurtzite GaN using the abinitio Boltzmann transport formalism, including all electron-phonon scattering processes and many-body quasiparticle band structures. We predict that the hole mobility can be increased by reversing the sign of the crystal-field splitting in such a way as to lift the split-off hole states above the light and heavy holes. We find that a 2% biaxial tensile strain can increase the hole mobility by 230%, up to a theoretical Hall mobility of 120 cm^{2}/V s at room temperature and 620 cm^{2}/V s at 100K.

Theoretical Exploration of Carrier Dynamics in Amorphous Pyrene-Fluorene Derivative Organic Semiconductors.

In this report, a series of amorphous organic optoelectronic pyrene–fluorene derivative materials (BP1, BP2, PFP1, PFP2, OP1, OP2) were systematically investigated through a theoretical method. Their molecular structures are different due to the difference of substitution groups at C9 of the fluorene core, which include electron-rich pyrene group (PFP1 and PFP2), relatively neutral phenyl group (BP1 and BP2), and electron-withdrawing oxadiazole group (OP1 and OP2). In the beginning, through the physical model analysis, this report proposes that the concept of p-type or n-type is not flawless because there is no real doping process in these molecular organic semiconductors. To prove such a concept, the Marcus theory and first-principles were employed to calculate the intrinsic transfer mobility of these materials. Not as the common method used for the single crystal, in this report, a series of disorderly designed lattice cells were constructed to represent the disordered distribution of the amorphous pyrenyl–fluorene derivatives. Then, the reorganization energy of materials was calculated by the adiabatic potential energy surface method. The transfer integral of dimers was calculated in possible hopping pathways near the central molecule. Research results show that the six pyrene–fluorene materials all possess intrinsic bipolar transfer characteristics. In addition, it is also showed that the electron-rich group is not necessary to improve hole transfer, and that the electron-withdrawing group is also not necessary to improve electron transfer.

Interplay between in-plane and flexural phonons in electronic transport of two-dimensional semiconductors

Out-of-plane vibrations are considered as the dominant factor limiting the intrinsic carrier mobility of suspended two-dimensional materials at low carrier concentrations. Anharmonic coupling between in-plane and flexural phonon modes is usually excluded from the consideration. Here we present a theory for the electron-phonon scattering, in which the anharmonic coupling between acoustic phonons is systematically taken into account. Our theory is applied to the typical group V two-dimensional semiconductors: hexagonal phosphorus, arsenic, and antimony. We find that the role of the flexural modes is essentially suppressed by their coupling with in-plane modes. At dopings lower than 10$^{12}$ cm$^{-2}$ the mobility reduction does not exceed 30\%, being almost independent of the concentration. Our findings suggest that compared to in-plane phonons, flexural phonons are considerably less important in the electronic transport of two-dimensional semiconductors, even at low carrier concentrations.

Exploring the optoelectronic and charge transport properties of Pechmann dyes as efficient OLED materials

The structural, optoelectronic, and charge transport properties of dimethyl 2,3-bis((4-(bis(4-methoxyphenyl)amino)phenyl)ethynyl)fumarate (parent compound; E-1) and its acid-induced double lactonization Pechmann dyes, i.e., (E)-5,5′-bis(4-(bis(4-methoxyphenyl)amino)phenyl)-2H,2′H-[3,3′ bifuranylidene]-2,2′-dione (P55), and 3,7-bis(4-(bis(4-methoxyphenyl)amino)phenyl)pyrano[4,3-c]pyran-1,5-dione (P66) were investigated by DFT and TD-DFT calculations for potential use as efficient materials for organic light-emitting diodes (OLEDs). The charge transfer behaviour of these dyes has also been explored by shedding light on the frontier molecular orbitals, vertical ionization potential, reorganization energy, excitation energies, transfer integrals and intrinsic mobility for holes and electrons. The experimental absorption and fluorescence spectra were successfully reproduced by TD-BMK/6-31+G(d,p) level with the conductor-like polarizable continuum model (CPCM) in toluene. Additionally, calculated hole intrinsic mobility of P66 was observed greater than the electron one revealing this compound as p-type material that is rational to experimental data. Moreover, charge transfer nature (p-type or n-type) of E-1 and P55 has been probed on the bases of reorganization energy, transfer integrals and intrinsic mobility which showed that both dyes would be better p-type materials.

Large-Pore Mesoporous CeO2 -ZrO2 Solid Solutions with In-Pore Confined Pt Nanoparticles for Enhanced CO Oxidation.

Active and stable catalysts are highly desired for converting harmful substances (e.g., CO, NOx ) in exhaust gases of vehicles into safe gases at low exhaust temperatures. Here, a solvent evaporation-induced co-assembly process is employed to design ordered mesoporous Cex Zr1- x O2 (0 ≤ x ≤ 1) solid solutions by using high-molecular-weight poly(ethylene oxide)-block-polystyrene as the template. The obtained mesoporous Cex Zr1- x O2 possesses high surface area (60-100 m2 g-1 ) and large pore size (12-15 nm), enabling its great capacity in stably immobilizing Pt nanoparticles (4.0 nm) without blocking pore channels. The obtained mesoporous Pt/Ce0.8 Zr0.2 O2 catalyst exhibits superior CO oxidation activity with a very low T100 value of 130 °C (temperature of 100% CO conversion) and excellent stability due to the rich lattice oxygen vacancies in the Ce0.8 Zr0.2 O2 framework. The simulated catalytic evaluations of CO oxidation combined with various characterizations reveal that the intrinsic high surface oxygen mobility and well-interconnected pore structure of the mesoporous Pt/Ce0.8 Zr0.2 O2 catalyst are responsible for the remarkable catalytic efficiency. Additionally, compared with mesoporous Pt/Cex Zr1- x O2 -s with small pore size (3.8 nm), ordered mesoporous Pt/Cex Zr1- x O2 not only facilitates the mass diffusion of reactants and products, but also provides abundant anchoring sites for Pt nanoparticles and numerous exposed catalytically active interfaces for efficient heterogeneous catalysis.

Effect of semiconductor surface homogeneity and interface quality on electrical performance of inkjet-printed oxide field-effect transistors

In semiconductor technology, the crystallite size of semiconductors is often directly correlated with their superior intrinsic and device mobility. However, when solution-processed, large crystals may bring in higher surface roughness and layer inhomogeneity, which can deteriorate the interface quality and device performance. Along this line, a thorough study on printed oxide field-effect transistors (FETs) has been performed, where the relative significance of crystallite size, surface roughness and spatial homogeneity are evaluated. The comprehensive investigations suggest the spatial homogeneity to be more important than crystallite size in solution processed/printed devices. It is demonstrated that the addition of a small amount of high boiling point polyol in the precursor ink can create large nucleation sites, resulting in reduced average crystallite size, superior inter-particle neck formation, and high spatial homogeneity. Interestingly, carefully estimated device mobility of these polyol-derived In2O3 FETs (∼50–55 cm2 V−1 s−1) is found to be larger than the FETs prepared without polyols, although the crystallite size of the former is an order of magnitude smaller. The high spatial homogeneity and the large mobility values of the polyol-derived In2O3 transistors, as compared to the amorphous oxide FETs, lowers the importance of the latter, at least within the solution-processed/printed electronics domain.

Charge carrier and optoelectronic properties of phenylimidazo[1,5-a]pyridine-containing small molecules at molecular and solid-state bulk scales

Various optoelectronic and charge transfer properties of 3-(1H-indol-3-yl)-1-phenylimidazo[1,5-a]pyridine (Comp 1), 1,3-diphenylimidazo[1,5-a]pyridine (Comp 2), and 3-(Anthracen-9-yl)-1-phenylimidazo[1,5-a]pyridine (Comp 3) have been explored at the molecular and solid state bulk level. The absorption and emission spectra (λabs and λemi) were calculated by time domain B3LYP/6-311G*, CAM-B3LYP/6-311G*, LC-BLYP/6-311G* and PBE0/6-311G* levels. The hybrid functionals B3LYP and PBE0 successfully reproduced experimental λabs and λemi more accurately than the long-range corrected functionals. Effect of indol, phenyl and anthracenyl moieties on various properties of interests has been explored. The indol and phenyl units lead to smaller hole reorganization energies and greater transfer integrals resulting superior hole intrinsic mobility for Comps 1 and 2. The substitution of anthracenyl moiety at phenylimidazo[1,5-a]pyridine core increase the electron affinity and electron transfer integrals while decrease the electron reorganization energy. The n- or p-type charge transfer nature of Comp 3 or Comps 1 and 2 has been explored, respectively. The frontier molecular orbitals, ionization potential, electron affinity, reorganization energy, transfer integral, intrinsic mobility, density of states, dielectric constant, extinction coefficient, refractive indices, reflectivity and conductivity shown that imidazo[1,5-a]pyridine derivatives would be efficient materials to be used in multifunctional organic semiconductor devices.

Exploiting Meta-Path with Attention Mechanism for Fine-Grained User Location Prediction in LBSNs

Based on the huge volumes of user check-in data in LBSNs, users’ intrinsic mobility patterns can be well explored, which is fundamental for predicting where a user will visit next given his/her historical check-in records. As there are various types of nodes and interactions in LBSNs, they can be treated as Heterogeneous Information Network (HIN) where multiple semantic meta-paths can be extracted. Inspired by the recent success of meta-path context based embedding techniques in HIN, in this paper, we design a deep neural network framework leveraging various meta-path contexts for fine-grained user location prediction. Experimental results based on two real-world LBSN datasets demonstrate the best effectiveness of the proposed approach using various evaluation metrics than others.

Balancing Light Absorption and Charge Transport in Vertical SnS2 Nanoflake Photoanodes with Stepped Layers and Large Intrinsic Mobility

AbstractSignificant optical absorption in the blue–green spectral range, high intralayer carrier mobility, and band alignment suitable for water splitting suggest tin disulfide (SnS2) as a candidate material for photo‐electrochemical applications. In this work, vertically aligned SnS2 nanoflakes are synthesized directly on transparent conductive substrates using a scalable close space sublimation (CSS) method. Detailed characterization by time‐resolved terahertz and time‐resolved photoluminescence spectroscopies reveals a high intrinsic carrier mobility of 330 cm2 V−1 s−1 and photoexcited carrier lifetimes of 1.3 ns in these nanoflakes, resulting in a long vertical diffusion length of ≈1 µm. The highest photo‐electrochemical performance is achieved by growing SnS2 nanoflakes with heights that are between this diffusion length and the optical absorption depth of ≈2 µm, which balances the competing requirements of charge transport and light absorption. Moreover, the unique stepped morphology of these CSS‐grown nanoflakes improves photocurrent by exposing multiple edge sites in every nanoflake. The optimized vertical SnS2 nanoflake photoanodes produce record photocurrents of 4.5 mA cm−2 for oxidation of a sulfite hole scavenger and 2.6 mA cm−2 for water oxidation without any hole scavenger, both at 1.23 VRHE in neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte.

Facile growth of AgVO3 nanoparticles on Mo-doped BiVO4 film for enhanced photoelectrochemical water oxidation

BiVO4 is largely restricted by its intrinsic poor electron mobility and sluggish surface reaction. Aiming at eliminating the deficiency of BiVO4, we assemble a novel composite photoanode through simultaneous bulk and surface modification. In detail, controllable Mo doping is manipulated through a spin-coating method, and AgVO3 nanoparticles are loaded by successive ionic layer adsorption and reaction (SILAR) process. The as-prepared photoanode boosts the photocurrent density by 7 folds at 1.23 V vs. RHE and moves the onset potential negatively by 387 mV compared to the bare BiVO4. Furthermore, the charge injection efficiency is substantially boosted to 49% at 1.23 VRHE, nearly 5 times as that of pure BiVO4. With detailed physical and electrochemical characterization, we reveal the essential reason for the boosted photoelectrochemical activity. Appropriate Mo doping can simultaneously enhance the carrier density and electron mobility of BiVO4. Specifically, more (0 4 0) facet are exposed for retarding charge recombination and increasing photocatalytic reaction sites. Then the simple SILAR process grows AgVO3 nanoparticles homogeneously with uniform diameter of 3–5 nm on BiVO4, forming p-n heterojunction to accelerate charge separation, and abundant oxygen vacancy emerged on the surface, which has been demonstrated as active sites for water oxidation reaction.

Modeling of efficient pyrene-core substituted with electron-donating groups as hole-transporting materials in perovskite solar cells

In perovskite solar cells (PSCs), the hole extraction/transport and the device stability are strongly dependent on the molecular structure of the hole transporting material (HTM). Designing of organic HTMs that are efficient for charge extraction and transport when integrated into optoelectronic devices is of great importance. The two fabricated devices: TiO2/CH3NH3PbI3/PYOMe/Au and TiO2/CH3NH3PbI3/spiro-OMeTAD/Au PSCs, where the HTMs in the two devices are a pyrene-core N1,N1,N3,N3,N6,N6,N8,N8-octakis(4-methoxyphenyl)pyrene-1,3,6,8-tetraamine (PYOMe) and the classical spiro-OMeTAD, respectively, recorded very comparable performance. The overall power conversion efficiencies are 12.4 and 12.7%, respectively. However, the preparation of PYOMe is more cost-effective, and it is predicted that the PY-core compounds will have faster charge-transport ability compared to the spirobifluorene core. Herein, we have engineered a PY-core compound (PYOMe) with groups of different electron-donating abilities at the para-position. DFT/M06/6-311G(d,p) and TDDFT/M06/6-31G(d) levels were used to investigate the geometrical, electronic and optical properties of these derivatives in their neutral, cationic and anionic forms. The frontier orbital energy levels and their spatial distribution were computed for all derivatives and compared in the neutral and ionic forms. The results indicated that groups with different electron-donating abilities in the PY-core framework lead to different optical properties. According to the calculations, stronger electron-donating substituents remarkably destabilize the HOMO and LUMO, while weaker electron-donating substituents stabilize them. The reorganization energy, electron affinity, and ionization potential were also calculated and discussed. Finally, to shed light on the intrinsic mobility, the charge transport properties of three HTMs as representative examples were calculated and analyzed.

Elastic behavior and intrinsic carrier mobility for monolayer SnS and SnSe: First-principles calculations

Abstract Two-dimensional (2D) SnS and SnSe like black phosphorus have recently attracted great attention for their potential applications in next-generation electronics. Here, first-principles calculations are employed to investigate uniaxial strain behaviors and carrier mobilities of these monolayer structures. They are more structurally stable in armchair strain ( y axis strain) than in zigzag strain ( x axis strain), because zigzag deformation of the latter can be spontaneously transformed into former. Band gaps exhibit abnormal behavior with armchair strain changing from e y = −5 % to e y = 15 % , which is elucidated through the orbital hybridization between p states of S or Se atoms and the p states of Sn atoms. Surprisingly, it found that hole mobility is mainly determined by longitudinal acoustic phonon scattering, while electron mobility is mainly dominated by optical phonon scattering. The mobility can be tuned by uniaxial strain. Tensile strain along armchair direction will reduce carrier mobility. At compressive strain of e y = −5 % , the mobility of SnSe is increased to μ x =1.40× 10 3 and μ y =2.35× 10 3 cm 2 V −1 s −1 for electron, and μ x = 1.31 × 10 3 and μ y = 1.46 × 10 3 cm 2 V −1 s −1 for hole. Excellent elastic property, moderate band gap and tunable carrier mobility make SnSe promising for application in nanoelectronics and optoelectronics.

Wurtzite phonons and the mobility of a GaN/AlN 2D hole gas

To make complementary GaN electronics a desirable technology, it is essential to understand the low mobility of 2D hole gases in III-Nitride heterostructures. This work derives both the acoustic and optical phonon spectra present in one of the most prominent p-channel heterostructures (the all-binary GaN/AlN stack) and computes the interactions of these spectra with the 2D hole gas, capturing the temperature dependence of its intrinsic mobility. Finally, the effects of strain on the electronic structure of the confined 2D hole gas are examined and a means is proposed to engineer the strain to improve the 2D hole mobility for enhanced p-channel device performance, with the goal of enabling wide-bandgap CMOS.

Electrical characterization of graphene source/drain electrodes in amorphous indium-gallium-zinc-oxide thin-film transistors subjected to plasma treatment in contact regions

In this paper, the electrical characteristics of a-IGZO TFTs with graphene source/drain (S/D) electrodes subjected to argon plasma treatment are analyzed. Depending on the channel length (L), the a-IGZO TFTs showed parasitic resistance dominant (L < 30 μm) and channel conduction dominant regions (L ≥ 30 μm). Using the transmission line method, the intrinsic parameters were extracted. The intrinsic field-effect mobility was about 9.79 cm2 V−1 s−1 and the width-normalized parasitic resistance value was about 460 Ω∙cm, which are comparable with those of a-IGZO TFTs having S/D plasma-treated regions with no contact metal. Temperature-dependent measurement indicates that the graphene electrodes affected the thermally deactivated behavior of the a-IGZO TFTs, which is different from the case of a-IGZO TFTs having conventional metal electrodes.

Improved electrical performance of multilayer MoS2 transistor by incorporating Al into host HfO2 as gate dielectric

The effects of incorporating Al into HfO2 as gate dielectric on the back-gated MoS2 transistor are investigated. Hf0.5Al0.5O as gate dielectric exhibits excellent electrical characteristics: on/off ratio of 1.8 × 107, carrier mobility of 49.3 cm2/(V · s), subthreshold swing of 118 mV dec−1, interface-state density of 2.3 × 1012 eV−1 cm−2, due to the Al incorporation into the HfO2 can passivate the defective states induced by oxygen vacancies and improve the dielectric densification and uniformity to obtain an excellent MoS2/Hf1-xAlxO interface. Besides, the intrinsic carrier mobility of 100 cm2 V−1 s−1 and contact resistance of 11.1 kΩ/μm indicates that the contact resistance is a main factor limiting the mobility.

Intrinsic transport properties of nanoporous graphene highly suitable for complementary field-effect transistors

The recent success to synthesize an ordered array of pores in graphene by a bottom-up approach (Moreno et al 2018 Science 360 199) yields a semiconducting nanoporous graphene with a bandgap of 0.6 eV. In this paper, we present calculations of the intrinsic carrier mobility in this new type of two-dimensional material. Using a fully atomistic approach, we show that carriers are mostly scattered by acoustic phonons, approximately like in semiconducting carbon nanotubes. The carrier mobility shows strong anisotropy and is as high as 800 cm2 (Vs)−1 at low carrier density. Such a high mobility, together with symmetric properties of electrons and holes, suggests that porous graphene is a promising candidate for next generations of complementary field-effect transistor technology.

Performance improvement in 4H-SiC(0001) p-channel metal-oxide-semiconductor field-effect transistors with a gate oxide grown at ultrahigh temperature

We fabricated 4H-SiC(0001) p-channel metal-oxide-semiconductor field-effect transistors (pMOSFETs) with gate oxide formed at 1600 °C under an oxygen partial pressure of 0.3 kPa. The fabricated pMOSFETs showed superior performance, suggesting suppression of carbon-related defects at the SiO2/SiC interface. A further performance improvement was achieved by subsequently employing forming gas annealing at 1200 °C. In particular, a peak field-effect mobility of up to 13 cm2V−1 s−1, 11% of the intrinsic hole mobility, was demonstrated. Moreover, the transistors had a stable threshold voltage up to 200 °C and high immunity against bias-temperature stress, suggesting improved reliability.

Theoretical study of charge-transport and optical properties of organic crystals: 4,5,9,10-pyrenedi-imides.

This work presents a systematic study of the conducting and optical properties of a family of aromatic di-imides reported recently and discusses the influences of side-chain substitution on the reorganization energies, crystal packing, electronic couplings and charge injection barrier of 4,5,9,10-pyrenedi-imide (PyDI). Quantum-chemical calculations combined with the Marcus-Hush electron transfer theory revealed that the introduction of a side chain into 4,5,9,10-pyrenedi-imide increases intermolecular steric interactions and hinders close intermolecular π-π stacking, which results in weak electronic couplings and finally causes lower intrinsic hole and electron mobility in t-C5-PyDI (μh = 0.004 cm2 V-1 s-1 and μe = 0.00003 cm2 V-1 s-1) than in the C5-PyDI crystal (μh = 0.16 cm2 V-1 s-1 and μe = 0.08 cm2 V-1 s-1). Furthermore, electronic spectra of C5-PyDI were simulated and time-dependent density functional theory calculation results showed that the predicted fluorescence maximum of t-C5-PyDI, corresponding to an S 1→S 0 transition process, is located at 485 nm, which is close to the experimental value (480 nm).