Giant Carrier Mobility in a Room-Temperature Ferromagnetic VSi2N4 Monolayer.

Using density functional theory (DFT), we investigate that two possible phases of VSi2N4 (VSN) may be realized, one called the "H phase" corresponding to what is known from calculation and herein the other new "T phase" being stabilized by a biaxial tensile strain of 3%. Significantly, the H phase is predicted to display a giant carrier mobility of 1 × 106 cm2 V-1 s-1, which exceeds that for most 2D magnetic materials, with a Curie temperature (TC) exceeding room temperature and a band gap of 2.01 eV at the K point. Following the H-T phase transition, the direct band gap shifts to the Γ point and increases to 2.59 eV. The Monte Carlo (MC) simulations also indicate that TC of the T phase VSN can be effectively modulated by strain, reaching room temperature under a biaxial strain of -4%. These results show that VSN should be a promising functional material for future nanoelectronics.

Electronic and Transport Properties of InSe/PtTe2 van der Waals Heterostructure.

Two-dimensional (2D) InSe and PtTe2 have drawn extensive attention due to their intriguing properties. However, the InSe monolayer is an indirect bandgap semiconductor with a low hole mobility. van der Waals (vdW) heterostructures produce interesting electronic and optoelectronic properties beyond the existing 2D materials and endow totally new device functions. Herein, we theoretically investigated the electronic structures, transport behaviors, and electric field tuning effects of the InSe/PtTe2 vdW heterostructures. The calculated results show that the direct bandgap type-II vdW heterostructures can be realized by regulating the stacking configurations of heterostructures. By applying an external electric field, the band alignment and bandgap of the heterostructures can also be flexibly modulated. Particularly, the hole mobility of the heterostructures is improved by 2 orders of magnitude to ∼103 cm2 V-1 s-1, which overcomes the intrinsic disadvantage of the InSe monolayer. The InSe/PtTe2 vdW heterostructures have great potential applications in developing novel optoelectronic devices.

Enhanced Direct Exchange Interaction and Hybridization by Single-Atom Linkers for High Curie Temperature and Superior Visible-Light Harvesting in Cr3(CN3)2.

Designing two-dimensional (2D) ferromagnetic (FM) semiconductors with elevated Curie temperature, high carrier mobility, and strong light harvesting is challenging but crucial to the development of spintronics with multifunctionalities. Herein, we show first-principles computation evidence of the 2D metal-organic framework Kagome ferromagnet Cr3(CN3)2. Monolayer Cr3(CN3)2 is predicted to be an FM semiconductor with a record-high Curie temperature of 943 K owing to the use of a single-atom linker (N), which results in strong direct d-p exchange interaction and hybridization between dyz/xz and pz of Cr and N, as well as excellent matching characteristics in energy and symmetry. The single-atom linker structural feature also leads to notable band dispersion and a relatively high carrier mobility of 420 cm2 V-1 s-1. Moreover, under the in-plane strain, 2D Cr3(CN3)2 can be tuned to possess a strong visible-light-harvesting functionality. These novel properties render monolayer Cr3(CN3)2 a distinct 2D ferromagnet with high potential for the development of multifunctional spintronics.

The high electron mobility for spin-down channel of two-dimensional spin-polarized half-metallic ferromagnetic EuSi2N4 monolayer.

The two-dimensional (2D) monolayer material MoSi2N4 was successfully synthesized in 2020[Hong et al., Science 369, 670, (2020)], exhibiting a plethora of new phenomena and unusual properties, with good stability at room temperature. However, MA2Z4 family monolayer materials involve primarily transition metal substitutions for M atoms. In order to address the research gap on lanthanide and actinide MA2Z4 materials, this work conducts electronic structure calculations on novel 2D MSi2N4 (M = La, Eu) monolayer materials by employing first-principles methods and CASTEP. High carrier mobility is discovered in the indirect bandgap semiconductor 2D LaSi2N4 monolayer (~5400 cm2 V-1 s-1) and in the spin (spin-down channel) carrier mobility of the half-metallic ferromagnetic EuSi2N4 monolayer (~2800 cm2 V-1 s-1). EuSi2N4 monolayer supplements research on spin carrier mobility in half-metallic ferromagnetic monolayer materials at room temperature and possesses a magnetic moment of 5 µB, which should not be underestimated. Furthermore, due to the unique electronic band structure of EuSi2N4 monolayer (with the spin-up channel exhibiting metallic properties and the spin-down channel exhibiting semiconductor properties), it demonstrates a 100% spin polarization rate, presenting significant potential applications in fields such as magnetic storage, magnetic sensing, and spintronics.

High Carrier Mobility and Promising Thermoelectric Module Performance of N-Type PbSe Crystals.

The scarcity of Te hampers the widespread use of Bi2Te3-based thermoelectric modules. Here, the thermoelectric module potential of PbSe is investigated by improving its carrier mobility. Initially, large PbSe crystals are grown with the temperature gradient method to mitigate grain boundary effects on carrier transport. Subsequently, light doping with <1mole‰ halogens (Cl/Br/I) increases room-temperature carrier mobility to ~1600 cm2 V-1 s-1, achieved by reducing carrier concentration compared to traditional heavy doping. Crystal growth design and light doping enhance carrier mobility without affecting effective mass, resulting in a high power factor ~40 µW cm-1 K-2 in PbSe-Cl/Br/I crystals at 300 K. Additionally, Cl/Br/I doping reduces thermal conductivity and bipolar diffusion, leading to significantly lower thermal conductivity at high temperature. Enhanced carrier mobility and suppressed bipolar effect boost ZT values across the entire temperature range in n-type PbSe-Cl/Br/I crystals. Specifically, ZT values of PbSe-Br crystal reach ~0.6 at 300 K, ~1.2 at 773 K, and the average ZT (ZTave) reaches ~1.0 at 300-773 K. Ultimately, ~5.8% power generation efficiency in a PbSe single leg with a maximum temperature cooling difference of 40 K with 7-pair modules is achieved. These results indicate the potential for cost-effective and high-performance thermoelectric cooling modules based on PbSe.

Elucidating the time-dependent charge neutrality point modulation of polymer-coated graphene field-effect transistors in an ambient environment.

The charge neutrality point (CNP) is one of the essential parameters in the development of graphene field-effect transistors (GFETs). For GFET with an intrinsic graphene channel layer, the CNP is typically near-zero-volt gate voltage, implying that a well-balanced density of electrons and holes exists in the graphene channel layer. Fabricated GFET, however, typically exhibits CNP that is either positively or negatively shifted from the near-zero-volt gate voltage, implying that the graphene channel layer is unintentionally doped, leading to a unipolar GFET transfer characteristic. Furthermore, the CNP is also modulated in time, indicating that charges are dynamically induced in the graphene channel layer. In this work, understanding and mitigating the CNP shift were attempted by introducing passivation layers made of polyvinyl alcohol and polydimethylsiloxane onto the graphene channel layer. The CNP was found to be negatively shifted, recovered back to near-zero-volt gate voltage, and then positively shifted in time. By analyzing the charge density, carrier mobility, and correlation between the CNP and the charge density, it can be concluded that positive CNP shifts can be attributed to the charge trapping at the graphene/SiO2interface. The negative CNP shift, on the other hand, is caused by dipole coupling between dipoles in the polymer layer and carriers on the surface of the graphene layer. By gaining a deeper understanding of the intricate mechanisms governing the CNP shifts, an ambiently stable GFET suitable for next-generation electronics could be realized.

Control over Charge Carrier Mobility in the Hole Transport Layer Enables Fast Colloidal Quantum Dot Infrared Photodetectors.

Solution-processed colloidal quantum dots (CQDs) are promising materials for photodetectors operating in the short-wavelength infrared region (SWIR). Devices typically rely on CQD-based hole transport layers (HTL), such as CQDs treated using 1,2-ethanedithiol. Herein, we find that these HTL materials exhibit low carrier mobility, limiting the photodiode response speed. We develop instead inverted (p-i-n) SWIR photodetectors operating at 1370 nm, employing NiOx as the HTL, ultimately enabling 4× shorter fall times in photodiodes (∼800 ns for EDT and ∼200 ns for NiOx). Optoelectronic simulations reveal that the high carrier mobility of NiOx enhances the electric field in the active layer, decreasing the overall transport time and increasing photodetector response time.

Self-Assembly Behavior, Aggregation Structure, and the Charge Carrier Transport Properties of S-Heterocyclic Annulated Perylene Diimide Derivatives.

The construction of high-performance n-type semiconductors is crucial for the advancement of organic electronics. As an attractive n-type semiconductor, molecular systems based on perylene diimide derivatives (PDIs) have been extensively investigated over recent years. Owing to the fascinating aggregated structure and high performance, S-heterocyclic annulated PDIs (SPDIs) are receiving increasing attention. However, the relationship between the structure and the electrical properties of SPDIs has not been deeply revealed, restricting the progress of PDI-based organic electronics. Here, we developed two novel SPDIs with linear and dendronized substituents in the imide position, named linear SPDI and dendronized SPDI, respectively. A series of structural and property characterizations indicated that linear SPDI formed a long-range-ordered crystalline structure based on helical supramolecular columns, while dendronized SPDI, with longer alkyl side chains, formed a 3D-ordered crystalline structure at a low temperature, which transformed into a hexagonal columnar liquid crystal structure at a high temperature. Moreover, no significant charge carrier transport signal was examined for linear SPDI, while dendronized SPDI had a charge carrier mobility of 3.5 × 10-3 cm2 V-1 s-1 and 2.1 × 10-3 cm2 V-1 s-1 in the crystalline and liquid crystalline state, respectively. These findings highlight the importance of the structure-function relationship in PDIs, and also offer useful roadmaps for the design of high-performance organic electronics for down-to-earth applications.

Wiring Covalent Organic Frameworks with Conducting Polymers.

Covalent organic frameworks are a class of crystalline porous polymers formed by linking organic units into periodically aligned skeletons and pores. Here we report a strategy for wiring these frameworks with conducting polymers via wall engineering and polymerization. We anchored each edge site with one pyrrole unit, which is densely packed along the z direction yet protruded from pore walls. This assembly enables the polymerization of pyrrole units to form polypyrrole and creates a new polypyrrole chain conformation. The resultant framework constitutes six single file polypyrrole chains in each pore and develop spatially segregated yet built-in single molecular wires with exceptional stable polarons. Hall effect measurements revealed that the materials are p-type semiconductors, increase conductivity by eight orders of magnitude compared to the pristine frameworks, and achieve a carrier mobility as large as 13.2 cm2 V-1 s-1. Our results open an avenue to π electronic frameworks by interlayer molecular wiring with conducting polymers.

High Thermoelectric Performance Related to PVDF Ferroelectric Domains in P-Type Flexible PVDF-Bi0.5 Sb1.5 Te3 Composite Film.

There is increasing demand to power Internet of Things devices using ambient energy sources. Flexible, low-temperature, organic/inorganic thermoelectric devices are a breakthrough next-generation approach to meet this challenge. However, these systems suffer from poor performance and expensive processing preventing wide application of the technology. In this study, by combining a ferroelectric polymer (Polyvinylidene fluoride (PVDF, ß phase)) with p-type Bi0.5 Sb1.5 Te3 (BST) a thermoelectric composite film with maximum is produced power factor. Energy filter from ferroelectric-thermoelectric junction also leads to high Seebeck voltage ≈242 µV K-1 . For the first time, compelling evidence is provided that the dipole of a ferroelectric material is helping decouple electron transport related to carrier mobility and the Seebeck coefficient, to provide 5× or more improvement in thermoelectric power factor. The best composition, PVDF/BST film with BST 95 wt.% has a power factor of 712 µW•m-1  K-2 . A thermoelectric generator fabricated from a PVDF/BST film demonstrated Pmax T 12.02 µW and Pdensity 40.8 W m-2 under 50 K temperature difference. This development also provides a new insight into a physical technique, applicable to both flexible and non-flexible thermoelectrics, to obtain comprehensive thermoelectric performance.

Superior Carrier Mobility Enabled by the Charge Channel Leads to Enhanced Thermoelectric Performance in BiCuSeO Composites.

BiCuSeO oxyselenides possess a highlighted thermoelectric performance among oxides, which originates from their intrinsically low thermal conductivity. However, intrinsic factors causing low thermal transport are also detrimental to carrier transport, leading to ultralow carrier mobility and relatively low electrical transport properties. Here, high-conductivity single-wall carbon nanotubes (SWCNTs) are adopted as the charge channels to be embedded in a BiCuSeO-based matrix, providing a transport pathway for charge carriers. The results show that carrier mobility is increased to 188 cm2 V-1 s-1 due to the SWCNTs composited, triggering an enhancement in electrical transport properties. Besides, the SWCNTs embedded in the matrix introduce abundant interfaces, suppressing phonon transport and depressing lattice thermal conductivity. With these achievements, a maximum zT of 0.84 at 818 K is realized in the composite with 0.1 wt% SWCNTs. The mechanical property of the composites is strengthened as well because of the SWCNTs. The work indicates that the SWCNTs, as the charge channels, propose an effective approach for enhancing carrier mobility in BiCuSeO-based materials, finally optimizing the thermoelectric performance as well as the mechanical property.

Single-Crystal Nanowire Cesium Tin Triiodide Perovskite Solar Cell.

This work reports for the first time a highly efficient single-crystal cesium tin triiodide (CsSnI3 ) perovskite nanowire solar cell. With a perfect lattice structure, low carrier trap density (≈5 × 1010 cm-3 ), long carrier lifetime (46.7 ns), and excellent carrier mobility (>600 cm2 V-1 s-1 ), single-crystal CsSnI3 perovskite nanowires enable a very attractive feature for flexible perovskite photovoltaics to power active micro-scale electronic devices. Using CsSnI3 single-crystal nanowire in conjunction with highly conductive wide bandgap semiconductors as front-surface-field layers, an unprecedented efficiency of 11.7% under AM 1.5G illumination is achieved. This work demonstrates the feasibility of all-inorganic tin-based perovskite solar cells via crystallinity and device-structure improvement for the high-performance, and thus paves the way for the energy supply to flexible wearable devices in the future.

Highly Soluble Ambipolar anti-Perylene-3,4 : 9,10-bis(benzimidazole)s Stabilize a Room-Temperature Columnar Hexagonal Phase.

A new series of highly soluble perylene anti-bis(4,5-dialkoxybenzimidazole)s bearing branched flexible chains stabilizing room temperature columnar hexagonal phase and with balanced ambipolar charge carrier mobility is reported for the first time. Only the anti isomer was successfully separated and characterized. These compounds have a high extinction coefficient, small optical band gap and wide absorption range, thus making them a promising class of ambipolar organic semiconductors capable of self-organizing.

Theoretical Study on Zigzag Boron Nitride Nanowires.

In this work, two kinds of BN-nanowires (BNnws): a-BNnw and d-BNnw, respectively composed of azo (N-N) and diboron (B-B) bonds, are proposed with the aid of the first-principles simulations. Their structural stabilities are carefully verified from the energetics, lattice dynamics, and thermodynamic perspectives. Similar to the other common boron nitride polymorph, the a-BNnw and d-BNnw are semiconductors with relatively wide band gaps of 3.256 and 4.631 eV at the HSE06 level, respectively. The corresponding projected DOS patterns point out that their band edges are composed of different atomic species, which can help with the separation of their excitons. The band gaps can be manipulated monotonically by axial strains within the elastic ranges. The major charge carriers are electron holes. Significantly, a-BNnw possesses very high carrier mobilities around 0.44×104  cm2 V-1 s-1 .

A study of 2D GeI2/InTe van der Waals hetero bilayer as a photocatalyst material.

The computational study of the van der Waals hetero (vdW) bilayer GeI2/InTe has been carried out in present study. The isolated monolayer GeI2and InTe have been studied first and the results were compared to the previous studies. The possible stackings are considered after the vdW interaction correction is applied in the structure relaxation. The vdW hetero bilayer stability has been checked from the phonon dispersion andab initioMolecular Dynamics calculations. The charge transfer from InTe to GeI2monolayer. Type-II indirect band gap (1.98, 2.01 eV) is verified by the projected band structure and band alignment calculations. The vdW hetero bilayer is a superior photocatalyst for the pH value up to pH = 0 to 11. The optical properties are calculated from the complex dielectric constant. The absorption coefficient shows the enhance absorption of light in the visible and ultraviolet regions. The vdW hetero bilayer has shown low reflectivity (37%) and a high refractive index (2.80) in the visible region. The enhanced optical properties have shown its possible applications in optoelectronic devices.

Impact of multi-domain effect on the effective carrier mobility of ferroelectric field-effect transistor.

HfO2-based ferroelectric field-effect transistors (FeFETs) are a promising candidate for multilevel memory manipulation and brain-like computing due to the multi-domain properties of the HfO2FE films based polycrystalline structure. Although there have been many reports on the working mechanism of the HfO2-based FeFET and improving its reliability, the impact of multi-domain effect on the effective carrier mobility (µchannel) has not been carried out yet. The effectiveµchanneldetermines the level of readout current and affects the accuracy of the precision of peripheral circuit. In this work, FeFETs with HfZrOxFE gate dielectric were fabricated, and the effect of write (or erase) pulses with linear gradient variation on the effectiveµchannelwas studied. For the multiple downward polarization under write pulses, theµchanneldegrades as the domains gradually switch to downward. This is mainly due to the enhancement of the scattering effect induced by the positive charges (e.g. oxygen vacanciesVO2+) trapping and the increase of channel carrier density. For the erase pulses, theµchannelincreases as the domains gradually reverse to upward, which is mainly due to the reduction of the scattering effect induced by the detrapping of positive charges and the decrease of channel carrier density. In addition, the modulation effect of multilevel polarization states onµchannelis verified by numerical simulation. This effect provides a new idea and solution for the development of low power HfO2-based FeFETs in neuromorphic computing.

Ligand-Engineered HgTe Colloidal Quantum Dot Solids for Infrared Photodetectors.

HgTe colloidal quantum dots (CQDs) are promising absorber systems for infrared detection due to their widely tunable photoresponse in all infrared regions. Up to now, the best-performing HgTe CQD photodetectors have relied on using aggregated CQDs, limiting the device design, uniformity and performance. Herein, we report a ligand-engineered approach that produces well-separated HgTe CQDs. The present strategy first employs strong-binding alkyl thioalcohol ligands to enable the synthesis of well-dispersed HgTe cores, followed by a second growth process and a final postligand modification step enhancing their colloidal stability. We demonstrate highly monodisperse HgTe CQDs in a wide size range, from 4.2 to 15.0 nm with sharp excitonic absorption fully covering short- and midwave infrared regions, together with a record electron mobility of up to 18.4 cm2 V-1 s-1. The photodetectors show a room-temperature detectivity of 3.9 × 1011 jones at a 1.7 µm cutoff absorption edge.

Photoconduction and Electroluminescence of Copper (II) Protoporphyrin and Chlorin Cu-C-e6.

Cu (II) protoporphyrin Cu-PP-IX and chlorin Cu-C-e6 were found to have both thin solid film formation and charge carrier transport abilities. In the layers deposited by resistive thermal evaporation, the mobilities of holes and electrons are on the order of 10-5 cm2 V-1 s-1. Organic light-emitting diodes incorporating the dye molecules as emitting dopants demonstrate electroluminescence in the UV and near-IR ranges.

Ultrahigh Carrier Mobility in Two-Dimensional IV-VI Semiconductors for Photocatalytic Water Splitting.

Two-dimensional materials have been developed as novel photovoltaic and photocatalytic devices because of their excellent properties. In this work, four δ-IV-VI monolayers, GeS, GeSe, SiS and SiSe, are investigated as semiconductors with desirable bandgaps using the first-principles method. These δ-IV-VI monolayers exhibit exceptional toughness; in particular, the yield strength of the GeSe monolayer has no obvious deterioration at 30% strain. Interestingly, the GeSe monolayer also possesses ultrahigh electron mobility along the x direction of approximately 32,507 cm2·V-1·s-1, which is much higher than that of the other δ-IV-VI monolayers. Moreover, the calculated capacity for hydrogen evolution reaction of these δ-IV-VI monolayers further implies their potential for applications in photovoltaic and nano-devices.

N=8 Armchair Graphene Nanoribbons: Solution Synthesis and High Charge Carrier Mobility.

Structurally defined graphene nanoribbons (GNRs) have emerged as promising candidates for nanoelectronic devices. Low band gap (<1 eV) GNRs are particularly important when considering the Schottky barrier in device performance. Here, we demonstrate the first solution synthesis of 8-AGNRs through a carefully designed arylated polynaphthalene precursor. The efficiency of the oxidative cyclodehydrogenation of the tailor-made polymer precursor into 8-AGNRs was validated by FT-IR, Raman, and UV/Vis-near-infrared (NIR) absorption spectroscopy, and further supported by the synthesis of naphtho[1,2,3,4-ghi]perylene derivatives (1 and 2) as subunits of 8-AGNR, with a width of 0.86 nm as suggested by the X-ray single crystal analysis. Low-temperature scanning tunneling microscopy (STM) and solid-state NMR analyses provided further structural support for 8-AGNR. The resulting 8-AGNR exhibited a remarkable NIR absorption extending up to ∼2400 nm, corresponding to an optical band gap as low as ∼0.52 eV. Moreover, optical-pump TeraHertz-probe spectroscopy revealed charge-carrier mobility in the dc limit of ∼270 cm2  V-1 s-1 for the 8-AGNR.