Superior Electron Transport Research Articles

Superior electron transport of ultra-thin SiC nanowires with one impending tensile monatomic chain

The effect of the tensile strain on the electron transport are studied theoretically for ultra-thin SiC Nanowires (NWs) with different diameters. Results show that these NWs exhibit distinctive and non-traditional stress-strain curves, characterized by multistage fluctuations. The tension greatly affects the electron transport. The strongest transmission appears when the wire is stretched to the near-emergence of the monatomic chain in the necking place (especially for thicker SiC NWs) which suppresses the electron transport, essentially originating from localized transmission eigenstates and pathways. This work provides an approach to enhance the electron transport and strong backing in promising applications of nanoelectronics and nano-piezoelectric devices for ultra-thin SiC NWs.

Gas sensors based on TiO2 nanostructured materials for the detection of hazardous gases: A review

Hazardous gases have been strongly associated with being a detriment to human life within the environment. The development of a reliable gas sensor with high response and selectivity is of great significance for detecting different hazardous gases. TiO 2 nanomaterials are promising candidates with great potential and excellent performance in gas sensor applications, such as hydrogen, acetone, ammonia, and ethanol detection. This review begins with a detailed discussion of the different dimensional morphologies of TiO 2 , which affect the gas sensing performance of TiO 2 sensors. The diverse morphologies of TiO 2 can easily be tuned by regulating the manufacturing conditions. Meanwhile, they exhibit unique characteristics for detecting gases, including large specific surface area, superior electron transport rates, extraordinary permeability, and active reaction sites, which offer new opportunities to improve the gas sensing properties. In addition, a variety of efforts have been made to functional TiO 2 nanomaterials to further enhance sensing properties, including TiO 2 -based composites and light-assisted gas sensors. The enhanced gas sensing mechanisms of multi-component composite nanomaterials based on TiO 2 include loaded noble metals, doped elements, constructed heterojunctions, and compounded with other functional materials. Finally, several studies have been summarized to demonstrate the comparative sensing properties of TiO 2 -based gas sensors.

Ligand exchange of SnO2 effectively improving the efficiency of flexible perovskite solar cells

Owing to the advantages of portability and flexibility, flexible perovskite solar cells have broad application prospects in the fields of portable wearable electronics and photovoltaic buildings. However, high temperature annealing process is often needed to improve the crystallinity and electron transport properties for electron transport layer, which restricts the development of flexible perovskite solar cells. In this work, well crystalline and monodisperse SnO2 nanocrystals are synthesized by facile solvothermal method with the assistance of oleic acid molecules. However, the presence of oleic acid molecules blocks the transportation of photogenerated electrons on account of the insulation of oleic acid molecules. To address the problem, the oleic acid ligands are ulteriorly peeled off by ligand exchange technique. Surprisingly, the resulted SnO2 electron transport layer exhibits superior electron extraction and transport capacity. The final power conversion efficiency comes to 17.96% from 13.71%.

Superior Mechanical Properties of GaAs Driven by Lattice NanotwinningSupported by the National Key Research and Development Program of China (Grant No. 2018YFA0703400), the National Natural Science Foundation of China (Grant Nos. 11704044 and 11974134), the Jilin Province Outstanding Young Talents Project (Grant No. 20190103040JH), and the China Postdoctoral Science Foundation (Grant No. 2018M631870).

Gallium arsenide (GaAs), a typical covalent semiconductor, is widely used in the electronic industry, owing to its superior electron transport properties. However, its brittle nature is a drawback that has so far significantly limited its application. An exploration of the structural deformation modes of GaAs under large strain at the atomic level, and the formulation of strategies to enhance its mechanical properties is highly desirable. The stress-strain relations and deformation modes of single-crystal and nanotwinned GaAs under various loading conditions are systematically investigated, using first-principles calculations. Our results show that the ideal strengths of nanotwinned GaAs are 14% and 15% higher than that of single-crystal GaAs under pure and indentation shear strains, respectively, without producing a significantly negative effect in terms of its electronic performance. The enhancement in strength stems from the rearrangement of directional covalent bonds at the twin boundary. Our results offer a fundamental understanding of the mechanical properties of single crystal GaAs, and provide insights into the strengthening mechanism of nanotwinned GaAs, which could prove highly beneficial in terms of developing reliable electronic devices.

Bidimensional H‐Bond Network Promotes Structural Order and Electron Transport in BPyMPMs Molecular Semiconductor

AbstractThe presence of a hydrogen bond (H‐Bond) network has been proved to impact significantly the efficiency of organic light‐emitting diode (OLED) devices by promoting molecular orientation and structural anisotropy in thin films. The design of specific compounds to control H‐Bond network formation in an amorphous material, and hence to improve OLED performances, is needed. A successful example is given by the bi‐pyridyl‐based family n‐type of organic semiconductors named BPyMPM. The experimental evidences demonstrate a surprisingly higher electron mobility in thin film composed of 4,6‐bis(3,5‐di(pyridine‐4‐yl)phenyl)‐2‐methylpyrimidine (B4PyMPM (B4)), which is almost two order of magnitude higher than mobility measured for very similar member of the family, 4,6‐bis(3,5‐di(pyridine‐2‐yl)phenyl)‐2‐methylpyrimidine (B2PyMPM (B2)). Herein, a comprehensive computational study is presented, wherein classical and ab initio methods are combined, to investigate the 2D H‐Bond network in B4 and B2 thin films. The results indicate that B4 forms a larger number of intermolecular C‐H···N H‐Bonds that promote a higher orientational and positional order in B4 films, and superior electron transport properties.

Spatial configuration engineering of perylenediimide-based non-fullerene electron transport materials for efficient inverted perovskite solar cells

Abstract Due to their excellent photoelectron chemical properties and suitable energy level alignment with perovskite, perylene diimide (PDI) derivatives are competitive non-fullerene electron transport material (ETM) candidates for perovskite solar cells (PSCs). However, the conjugated rigid plane structure of PDI units result in PDI-based ETMs tending to form large aggregates, limiting their application and photovoltaic performance. In this study, to restrict aggregation and further enhance the photovoltaic performance of PDI-type ETMs, two PDI-based ETMs, termed PDO-PDI2 (dimer) and PDO-PDI3 (trimer), were constructed by introducing a phenothiazine 5,5-dioxide (PDO) core building block. The research manifests that the optoelectronic properties and film formation property of PDO-PDI2 and PDO-PDI3 were deeply affected by the molecular spatial configuration. Applied in PSCs, PDO-PDI3 with three-dimensional spiral molecular structure, exhibits superior electron extraction and transport properties, further achieving the best PCE of 18.72% and maintaining 93% of its initial efficiency after a 720-h aging test under ambient conditions.

Solution-processed Ga-TiO2 electron transport layer for efficient inverted organic solar cells

To boost the charge transport and collection processes in inverted organic solar device, gallium (Ga) was added to conventional TiO2 film to form the Ga-TiO2 film. Compared to the pristine film, the Ga-TiO2 film showed better energy level alignment, which is in good agreement with the heightened open-circuit voltage and fill factor. Attributed to the superior electron transport, the Ga-TiO2 based solar cell achieved paramount power conversion efficiency (PCE) of 7.72%, while the pristine TiO2 based solar cell showed lower PCE of 6.65%.

Critical evidence for direct interspecies electron transfer with tungsten-based accelerants: An experimental and theoretical investigation

Accelerants can significantly enhance the biodegradability in anaerobic digestion (AD), which can be attributed to the direct interspecies electron transfer (DIET) mechanism. However, critical evidence for DIET mechanism is absent. In this work, nano-scale tungsten (W)-based compounds (WC, W2N, and W18O49) are employed to clarify the roles of W-based accelerants in AD systems. A DIET mechanism based on the W-based accelerants is proposed, and three critical pieces of evidence are provided: (i) First-principle density functional theory calculations provide theoretical evidence, illustrating that W-based accelerants are of zero band gap. (ii) Electrical conductivity evaluation further elucidates that W-based accelerants have superior electronic transport. (iii) Pyrosequencing of 16S rRNA gene confirms the existence of acetogens and methanogens in AD systems, which can act as electron-donor bacteria and electron-acceptor archaea, respectively. Combining theoretical with experimental results, the critical evidence provides a general strategy for understanding the DIET mechanism of accelerant in AD systems.

A carbon-doped tantalum dioxyfluoride as a superior electron transport material for high performance organic optoelectronics

The design and development of novel materials with superior charge transport capabilities plays an essential role for advancing the performance of electronic devices. Ternary and doped oxides can be potentially explored because of their tailored electronic energy levels, exceptional physical properties, high electrical conductivity, excellent robustness and enhanced chemical stability. Here, a route for improving metal oxide characteristics is proposed by preparing a novel ternary oxide, namely, carbon-doped tantalum dioxyfluoride (TaO2FCx) through a straightforward synthetic route and exploring its effectiveness as an electron transport material in optoelectronic devices based on organic semiconductors. Among other devices, we fabricated fluorescent green organic light emitting diodes with current efficiencies of 16.53 cd/A and single-junction non-fullerene organic solar cells reaching power conversion efficiencies of 14.14% when using the novel oxide as electron transport material. Our devices also exhibited the additional advantage of high operational and temporal stability. Non-fullerene OSCs based on the novel compound showed unprecedented stability when exposed to UV light in air due to the non-defective nature of TaO2FCx. We employed a tank of experiments combined with theoretical calculations to unravel the performance merits of this novel compound. This study reveals that properly engineered ternary oxides, in particular, TaO2FCx or analogous materials can enable efficient electron transport in organic optoelectronics and are proposed as an attractive route for the broader field of optoelectronic devices including metal-organic perovskite, colloidal quantum dot and silicon optoelectronics.

Constructed Ag NW@Bi/Al core-shell nano-architectures for high-performance flexible and transparent energy storage device.

Flexible and transparent energy storage devices (FTESDs) have recently attracted much attention for use in wearable and portable electronics. Herein, we developed an Ag nanowire (NW) @Bi/Al nanostructure as a transparent negative electrode for FTESDs. In the core-shell nanoarchitecture, the Ag NW percolation network with excellent conductivity contributes superior electron transport pathways, while the unique nanostructure provides an effective interface contact between the current collector and electroactive material. As a result, the electrode delivers a high capacity of 12.36 mF cm-2 (3.43 μA h cm-2) at 0.2 mA cm-2. With a minor addition of Al, the coulombic efficiency of the electrode remarkably increases from 65.1% to 83.9% and the capacity retention rate improves from 53.8% to 91.9% after 2000 cycles. Moreover, a maximum energy density of 319.5 μW h cm-2 and a power density of 27.5 mW cm-2 were realized by an interdigital structured device with a transmittance of 58% and a potential window of 1.6 V. This work provides a new negative electrode material for high-performance FTESDs in the next-generation integrated electronics market.

Potassium salt promoted regioselective three-component coupling synthesis of 1,4-asymmetrical [60]fullerene bisadducts with superior electron transport properties.

An efficient one-pot three-component domino coupling reaction of phenols, C60, and bromoalkanes was developed, resulting in the highly regioselective synthesis of 1,4-asymmetrical C60 bisadducts. The reaction utilizes KOtBu as a promoter and likely proceeds by an oxyanion/carbanion rearrangement/nucleophilic addition cascade. This new methodology is particularly effective for the synthesis of 1,4-asymmetrical C60 electron transport materials. Its utility is demonstrated by the synthesis of a new efficient 1,4-C60 ETM, which possesses better performance, easier synthesis, and a lower cost compared with the commercially available PCBM.

Superior Thermoelectric Performance of Ordered Double Transition Metal MXenes: Cr2TiC2T2 (T = -OH or -F).

Using SCAN-rVV10+U, we show Cr2TiC2 and Cr2TiC2T2 (T = -F and -OH) MXenes are moderate band gap semiconductors mostly in the antiferromagnetic state. All investigated MXene structures show large Seebeck coefficients (>400 μV/K), especially Cr2TiC2 (>800 μV/K) and Cr2TiC2F2 (>700 μV/K). The hole relaxation time of p-type Cr2TiC2(OH)2 is found to be ∼8 ps, ensuring its superior electron transport properties in comparison to other investigated MXenes. It is also discovered that the surface functionalization could decrease the phonon thermal conduction and that Cr2TiC2(OH)2 has the smallest lattice thermal conductivity (∼6.5 W/m·K) and the largest electron thermal conduction (>50 W/m·K with n = 1019 cm-3). We predict the ZT value of p-type Cr2TiC2(OH)2 can reach 3.0 at 600 K with the maximum thermoelectric conversion efficiency of 20%. Overall, the thermoelectric property of Cr-based ordered double transition metal MXenes is far superior to that of any known two-dimensional structures in the MXene family.

(Invited) Two-Dimensional Layered Materials Toward Phase-Engineered Hybrid Films

2D materials have attracted much attention because of frontier electronic materials due to its superior electronic transport properties and mechanical flexibility in the future, making it a potential material for high performance and wearable electronics. Graphene is a typical 2D materials with high carrier mobility; however, it still cannot be applied in transistor due to the lack of bandgap. A new type of 2D semiconducting materials called transition metal dichalcogenides (TMDCs), which are layered structure with the strong in-plane bonding and weak out-of-plane interactions similar to graphite, have been intensively studied. Recent studies have predicted exceptional physical properties upon reduced dimensionality attracting lots of attention due to the versatile physical chemical behaviors. Nevertheless, the synthesis and the study of the fundamental physical properties of TMDs are still in early stages. The lack of a large-area and reliable synthesis method restrict exploring all the potential applications of the TMDs. Chemical vapor deposition (CVD) is a traditional approach for the growth of TMDs; nevertheless, the high growth temperature is a major drawback for its to be applied in flexible electronics. In this talk, an inductively coupled plasma (ICP) was used to synthesize Transition Metal Dichalcogenides (TMDs) through a plasma-assisted selenization process of metal oxide (MOx) at a low temperature, as low as 250 °C. Compared to other CVD processes the use of ICP facilitates the decomposition of the precursors at lower temperatures; therefore, the temperature required for the formation of TMDs can be drastically reduced. WSe2 was chosen as a model material system due to its technological importance as a p-type inorganic semiconductor with an excellent hole mobility. Large-area synthesis of WSe2 on polyimide (30 x 40 cm2) flexible substrates and 8-inch silicon wafers with good uniformity was demonstrated at the formation temperature of 250 °C as confirmed by Raman and X-ray Photoelectron (XPS) spectroscopy. Furthermore, by controlling different H2/N2 ratios, hybrid WOx/WSe2 films can be formed at the formation temperature of 250 ºC as shown by TEM and confirmed by XPS. The applications including (1) water splitting, (2) gas sensors and (3) photodetectors will be reported.

Predicting three-dimensional icosahedron-based boron B60

The icosahedron-based bulk boron structures have extremely chemical and structural complexity, and are usually semiconductors at ambient conditions. Here we predict bulk boron phases with a 60-atom orthorhombic unit cell from an ab initio evolutionary structure search, termed as ${\mathrm{B}}_{60}$. The metastable structures can be either a conductor or a semimetal depending on their interstitial atomic positions. In particular, an orthorhombic structure with $Pnma$ symmetry ($Pnma\ensuremath{-}{\mathrm{B}}_{60}$), consisting of ${\mathrm{B}}_{12}$ icosahedra and twisted interstitial two-atom wide boron ribbons, is identified to be a topological node-line semimetal with potential superior electronic transport. The band structure and simulated electron-diffraction pattern of $Pnma\ensuremath{-}{\mathrm{B}}_{60}$ are in satisfactory agreement with the experimental data, suggesting that it may exist in the form of nanomaterials.

Syngas production by electrocatalytic reduction of CO2 using Ag-decorated TiO2 nanotubes

Huge efforts have been done in the last years on electrochemical and photoelectrochemical reduction of CO2 to offer a sustainable route to recycle CO2. A promising route is to electrochemically reduce CO2 into CO which, by combination with hydrogen, can be used as a feedstock to different added-value products or fuels. Herein, perpendicular oriented TiO2 nanotubes (NTs) on the electrode plate were grown by anodic oxidation of titanium substrate and then decorated by a low loading of silver nanoparticles deposited by sputtering (i.e. Ag/TiO2 NTs). Due to their quasi one-dimensional arrangement, TiO2 NTs are able to provide higher surface area for Ag adhesion and superior electron transport properties than other Ti substrates (e.g. Ti foil and TiO2 nanoparticles), as confirmed by electrochemical (CV, EIS, electrochemical active surface area) and chemical/morphological analysis (FESEM, TEM, EDS). These characteristics together with the role of the TiO2 NTs to enhance the stability of CO2·- intermediate formed due to titania redox couple (TiIV/TiIII) lead to an improvement of the CO production in the Ag/TiO2 NTs electrodes. Particular attention has been devoted to reduce the loading of noble metal in the electrode(14.5 %w/%w) and to increase the catalysts active surface area in order to decrease the required overpotential.

Multi-dimensional anatase TiO2 materials: Synthesis and their application as efficient charge transporter in perovskite solar cells

Recently, organo-metal halide perovskite (e.g., CH3NH3PbI3) solar cells have shown a significant surge in progress and efficiency. The photovoltaic performance of perovskite solar cells (PSC) extensively depends on the morphology of the materials at nano-level, because the intrinsic electrical, optical and electrochemical properties also change with a change in morphology. Different TiO2 nanostructures with diverse dimensionalities, 0D hollow TiO2 nanoparticles (HTNPs), 3D hollow TiO2 mesospheres (HTMSs) and 3D hierarchical TiO2 spheres (HTSs) were synthesized hydrothermally. The PSCs based on 3D HTSs or 3D HTMSs as electron transport layers (ETLs) shows better performance than the PSCs fabricated with 0D HTNPs as ETL. The enhanced results can be attributed to the better penetration of the perovskite materials in the porous network of the HTSs or HTMSs and decreased interfacial recombination due to superior charge extraction and electron transport at the HTSs/CH3NH3PbI or HTMSs/CH3NH3Pb interfaces. By using a self-assembled TiO2 compact layer (c-TiO2), followed by a mesoporous layer of HTSs, produces PSCs with an excellent efficiency of 15.08%. Such c-TiO2-ETL films may improve PSCs performance by enhancing electron extraction and block the photogenerated holes.

Two-dimensional MoS2-MoSe2 lateral superlattice with minimized lattice thermal conductivity

Single-layer transition metal dichalcogenides (TMDCs) are showing promising thermoelectric applications due to their superior stability and electronic transport properties. Unfortunately, the intrinsic high lattice thermal conductivity prevents their further improvement of thermoelectric performance. Motivated by recent experimental synthesis of two-dimensional TMDC heterostructures and superlattices, we propose to minimize the lattice thermal conductivity of single-layer MoS2 and MoSe2 using the lateral superlattice (LS) as building blocks. First-principles calculations with the phonon Boltzmann transport equation reveal a remarkably low lattice thermal conductivity of MoS2-MoSe2 LS due to the enhanced anharmonic phonon scattering as compared to the individual single-layer. We also show that the strong phonon anisotropy of MoS2-MoSe2 LS is primarily ascribed to the out-of-plane quadratic acoustic branch and orientation-dependent anharmonic scattering. Our calculations clearly demonstrate the advantages of LS structure in minimizing the lattice thermal conductivity of single-layer TMDCs and also accelerate their related applications in the field of renewable energy.

One-dimension TiO2 nanostructures with enhanced activity for CO2 photocatalytic reduction

The one dimension (1D) TiO2 nanotubes and nanorods were successfully prepared via a one-step hydrothermal method. The structure and properties of the catalysts were characterized by means of X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), Electron paramagnetic resonance (EPR), N2 adsorption-desorption, UV–vis diffuse reflectance and photoluminescence (PL) spectroscopy. Meanwhile, the photoelectrochemical properties were recorded through Mott-Schottky, transient photo-current responses and electrochemical impedance spectroscopy curves (EIS). The results showed that 1D TiO2 nanostructures catalysts had a high specific surface area, enhanced visible light response and superior electron transport properties. The photocatalytic activities of catalysts for reduction CO2 were investigated under 9 h irradiation of a 300 W Xe arc lamp equipped with UV 420 nm bandpass filter. The maximum yields of CH4 over the TiO2 nanotubes (TNT) and TiO2 nanorods (TNR) were 19.16 μmol/g.cat and 12.71 μmol/g.cat, respectively, which were approximately 2.33 folds and 1.48 folds higher than TiO2 nanoparticles (TNP). The enhanced photocatalytic activity of TiO2 nanotubes and nanorods could be ascribed to the presence of oxygen vacancies and defects formed during the calcination process and its special structures, which accelerates the transfer of electrons. Besides, the better activity of TNT than TNR may be ascribed to its unique hollow tubular structure and larger surface area.

High-performance inverted two-dimensional perovskite solar cells using non-fullerene acceptor as electron transport layer

Ruddlesden–Popper type two-dimensional (2D) and quasi-2D perovskite-based photovoltaics have been attracting increasing attention due to their high power conversion efficiency (PCE) and long-term stability. In this work, we fabricated inverted 2D perovskite solar cells (PSCs) using a non-fullerene acceptor as electron transport material. Employing a 55-nm thick 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthienyl)-dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC-Th) as electron transport layer, an average PCE of 8.4% was achieved for the 2D PSCs, which was comparable to that of 2D PSCs based on phenyl-C61-butyric acid methyl ester (PCBM). However, the long-term stability of the 2D PSCs was improved as the degradation of PCE was reduced from 19.8 to 14.6% following a 15-day duration, which can be attributed to the strong hydrophobic property of the ITIC-Th. The best sample based on the 55-nm thick ITIC-Th exhibited a high PCE of 8.9% with a stable power output and negligible hysteresis, indicating the superior electron transport property of ITIC-Th. Our results demonstrate that ITIC-Th is an effective alternative to PCBM to fabricate highly efficient and stable 2D PSCs.

(Invited) Large-Scale Growth of Two-Dimensional Materials Toward Phase-Engineered Hybrid Films Utilizing Low Temperature Plasma-Assisted Chemical Vapor Reaction Method

2D materials have attracted much attention because of frontier electronic materials due to its superior electronic transport properties and mechanical flexibility in the future, making it a potential material for high performance and wearable electronics. Graphene is a typical 2D materials with high carrier mobility; however, it still cannot be applied in transistor due to the lack of bandgap. A new type of 2D semiconducting materials called transition metal dichalcogenides (TMDCs), which are layered structure with the strong in-plane bonding and weak out-of-plane interactions similar to graphite, have been intensively studied. Recent studies have predicted exceptional physical properties upon reduced dimensionality attracting lots of attention due to the versatile physical chemical behaviors. Nevertheless, the synthesis and the study of the fundamental physical properties of TMDs are still in early stages. The lack of a large-area and reliable synthesis method restrict exploring all the potential applications of the TMDs. Chemical vapor deposition (CVD) is a traditional approach for the growth of TMDs; nevertheless, the high growth temperature is a major drawback for its to be applied in flexible electronics. In this talk, an inductively coupled plasma (ICP) was used to synthesize Transition Metal Dichalcogenides (TMDs) through a plasma-assisted selenization process of metal oxide (MOx) at a low temperature, as low as 250 °C. Compared to other CVD processes the use of ICP facilitates the decomposition of the precursors at lower temperatures; therefore, the temperature required for the formation of TMDs can be drastically reduced. WSe2 was chosen as a model material system due to its technological importance as a p-type inorganic semiconductor with an excellent hole mobility. Large-area synthesis of WSe2 on polyimide (30 x 40 cm2) flexible substrates and 8-inch silicon wafers with good uniformity was demonstrated at the formation temperature of 250 °C as confirmed by Raman and X-ray Photoelectron (XPS) spectroscopy. Furthermore, by controlling different H2/N2 ratios, hybrid WOx/WSe2 films can be formed at the formation temperature of 250 ºC as shown by TEM and confirmed by XPS. The applications o including (1) water splitting, (2) gas sensors, (3) photodetectors, (3) anode materials in secondary ion battery will be reported.