Ultrafast Charge Transport Research Articles

3D-printed film architecture via automatic micro 3D-printing system: Micro-intersection engineering of V2O5 thin/thick films for ultrafast electrochromic energy storage devices

Existing approaches to develop electrochromic (EC) energy storage devices typically focus on the modification of active materials toward a porous morphology, crystallinity, heteroatom-doping, and hybridization/composites with functional materials; however, these approaches may be unsuitable for the full commercialization of EC energy storage devices owing to their complex and expensive processes and incompatibility with large-scale and mass production. Herein, we fabricate 3D-printed film architectures for ultrafast EC energy storage devices, including micro-intersections of vanadium oxide (VO) thin/thick films, via an automatic micro 3D-printing system without active material modification. The micro-intersection originates from the one-pot 3D-printing of vertical lines, creating VO grid-micropatterns with nanoscale thin/thick films on fluorine-doped tin oxide (FTO)/glass. The micro-intersection density was engineered by adjusting the number of printed-lines per unit area and optimized micro-intersection density (OD-3DVO) allows enhanced behaviors of electrons and Li-ions as follows: (i) the thin film architecture alleviates rate-limiting processes that produce sluggish electron transfer and ionic diffusion kinetics by shortening transport channels between FTO and the electrolyte, thereby accelerating ultrafast charge transport. (ii) Micro-width/nano-depth 3D-wells induce capillary force pumping and deep electrolyte infiltration at the interface, thereby enhancing electrolyte wettability. (iii) grid-micropatterned film morphology provides extensive redox sites and high visible transmissivity; thus, the OD-3DVO active layers in EC energy storage devices featured simultaneously enhanced optical and energy storage performances, especially ultrafast switching speeds (0.7 and 0.9 s for coloration and bleaching) and record-high areal energy density at high areal power density (14.03 µWh/cm2 at 25 mW/cm2).

High-Performance Broad-Band Photodetection Based on Graphene-MoS2xSe2(1-x) Alloy Engineered Phototransistors.

The concept of alloy engineering has emerged as a viable technique toward tuning the band gap as well as engineering the defect levels in two-dimensional transition-metal dichalcognides (TMDCs). The possibility of synthesizing these ultrathin TMDC materials through a chemical route has opened up realistic possibilities to fabricate hybrid multifunctional devices. By synthesizing nanosheets with different composites of MoS2xSe2(1-x) (x = 0 - 1) using simple chemical methods, we systematically investigate the photoresponse properties of three terminal hybrid devices by decorating large-area graphene with these nanosheets (x = 0, 0.5, 1) in 2D-2D configurations. Among them, the graphene-MoSSe hybrid phototransistor exhibits optoelectronic properties superior to those of its binary counterparts. The device exhibits extremely high photoresponsivity (>104 A/W), low noise equivalent power (∼10-14 W/Hz0.5), and higher specific detectivity (∼1011 jones) in the wide UV-NIR (365-810 nm) range with excellent gate tunability. The broad-band light absorption of MoSSe, ultrafast charge transport in graphene, and controllable defect engineering in MoSSe makes this device extremely attractive. Our work demonstrates the large-area scalability with the wafer-scale production of MoS2xSe2(1-x) alloys, having important implications toward the facile and scalable fabrication of high-performance optoelectronic devices and providing important insights into the fundamental interactions between van der Waals materials.

Ultrafast charge transfer enhanced nonlinear optical properties of CH3NH3PbBr3 perovskite quantum dots grown from graphene

Abstract Halide perovskite quantum dots (PQDs) have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor materials thanks to their peculiar physical and electronic structures. By further improving the nonlinear optical properties of PQDs, it is expected to adapt to ultrafast photonics applications. This work reported the nonlinear optical properties of methylammonium lead bromide-graphene (CH3NH3PbBr3-G) composites synthesized by growing CH3NH3PbBr3 quantum dots directly from a graphene oxide lattice. Our experiments indicate that the combined advantages of the ultrafast charge transport properties from graphene and the strong charge generation efficiency of perovskite can be integrated together. The CH3NH3PbBr3-G composite exhibited enhanced saturable absorption properties with large modulation depth and very low saturation intensity. The transient absorption spectra and carrier dynamics analysis revealed that the enhancement of the saturated absorption properties of the composites mainly arose from the ultrafast charge transfer between G and CH3NH3PbBr3 which promoted the coupling between different states. The results pave the way for the design of optical switches or mode lockers based on saturable absorbers with good performance.

High sensitivity and fast response infrared detector fabricated with the Bi2Se3-PbSe heterojunction

A heterojunction photodetector based on the photogating effect is fabricated by combining the novel topological insulator Bi2Se3 with the traditional narrow-gap PbSe. The Bi2Se3-PbSe heterojunction is constructed by depositing PbSe thin film on surface of Bi2Se3 with the molecular beam epitaxy technique. Large lattice mismatch between cubic PbSe and rhombohedral Bi2Se3 is relaxed through the van der Waals epitaxial growth process, leading to an atomically abrupt and smooth Bi2Se3-PbSe heterointerface. Benefiting from the prominent photosensitivity in narrow-gap PbSe and the ultrafast charge transport in topological insulator Bi2Se3, high photosensitivity and fast response are simultaneously observed in the as-fabricated photodetectors. Responsivity and detectivity are 31.4 AW−1 and 1.7 × 1011 cmHz0.5 W−1 respectively. Rising and falling time are 240 μs and 500 μs respectively. In this structure, the appropriate band alignment of Bi2Se3-PbSe heterojunction and the photogating effect could synergistically utilize the advantages of both Bi2Se3 and PbSe, suggesting a potential solution to broaden the application prospect for both sides.

Dual carbon composited with Co9S8 through C-S bond as a high performance Binder-Free anode for Sodium-Ion batteries

Cobalt sulfide/ carbon cloth composites with conversion reactions mechanism as anodes for sodium-ion storage have attracted tremendous attention due to high theoretical capacity and binder-free. However, the severe volume change and exfoliation of cobalt sulfide makes composite materials cycle stability and rate capability unsatisfactory. In this work, we report dual carbon materials composited with Co9S8 through C-S bonds to further stabilize the structure of the composite (Co9S8/CC@C). Dual C-S bonds can provide ultrafast charge transport paths between Co9S8 and carbon materials, further ensure the stable reversible conversion reaction process. Furthermore, dual C-S bonds effectively prevent exfoliation of Co9S8 from carbon cloth, significantly improved structural stability of the composite. Benefiting from dual C-S bonds, the Co9S8/CC@C composite electrode delivers high reversible capacity of 785.6 mAh g−1 at 0.1 A g−1 after 100 cycles with capacity retention of 74.81 %. Notably, it preserves a considerable capacity of 417.2 mAh g−1 after 1000 cycles at 5 A g−1. This work provides a good reference for improving cycle stability and rate performance of Co9S8-based electrodes to designing advanced anodes for sodium-ion batteries.

Optical-Field-Driven Electron Tunneling in Metal-Insulator-Metal Nanojunction.

Optical‐field driven electron tunneling in nanojunctions has made demonstrable progress toward the development of ultrafast charge transport devices at subfemtosecond time scales, and have evidenced great potential as a springboard technology for the next generation of on‐chip “lightwave electronics.” Here, the empirical findings on photocurrent the high nonlinearity in metal–insulator–metal (MIM) nanojunctions driven by ultrafast optical pulses in the strong optical‐field regime are reported. In the present MIM device, a 14th power‐law scaling is identified, never achieved before in any known solid‐state device. This work lays important technological foundations for the development of a new generation of ultracompact and ultrafast electronics devices that operate with suboptical‐cycle response times.

Spontaneously Forming Oxide Layer of High Entropy Alloy Nanoparticles Deposited on Porous Carbons for Supercapacitors

AbstractHigh‐entropy alloy nanoparticles with a thin metal oxide layer, deposited onto hypercrosslinked polymer‐based carbon (HCPC), were synthesized through the adsorption of five metal ions in the hypercrosslinked polymers and subsequent in situ reduction of such ions (Fe2+, Co2+, Ni2+, Cu2+ and Sn2+), followed by a carbonization process. The as‐prepared composites exhibit an excellent specific capacitance (495.4 F/g at 0.5 A/g in 1 M KOH), a superior cycling stability (94.7 % of initial capacitance after 15000 cycles at 10 A/g), and ultrafast charge transport kinetics (88.9 % fast kinetic capacitance at 200 mV/s). These excellent electrochemical behaviors are attributed to the pseudocapacitance of the metal oxide layer spontaneously formed on the outermost surface of the composite, having a thickness at the nanometer scale. Such findings suggest that this simple strategy, combining multimetallic nanoparticles with porous carbon materials, has promising potential to improve specific capacitance and paves the way to develop both the preparation of high‐entropy alloys and supercapacitors with improved performance.

I–V characteristics of an atomically thin graphene-boron nitride heterostructure

Through recent advancement in nanofabrication techniques, the complex interfaces between different materials have been routinely fabricated and intensely studied. In this context, two-dimensional van der Waals heterostructures have attracted great interest from both fundamental and applied perspectives due to its potential applications in next-generation optoelectronic devices. In this work, using a quantum mechanical method based on real-time-propagation time dependent density functional theory (rt-TDDFT) simulations, we study the ultrafast charge transport dynamics and I–V characteristics of vertically stacked graphene/h-BN heterostructure subjected to external bias voltage in the range of 0.25 to 2 eV. Among other insights, our study demonstrates ultrafast charge transport dynamics at the sub-femtosecond time scale and distinctive I–V characteristics analogous to a parallel plate capacitor.

Current Amplification and Ultrafast Charge Transport in a Single Microdroplet of Bromide/Polybromide-Based Ionic Liquid

Bromide/polybromide-based ionic liquids have recently gained attention as energy storage devices because of their dual roles as a solvent and a redox pair. However, their redox reaction is accompan...

Design strategies for two‐dimensional material photodetectors to enhance device performance

AbstractTwo‐dimensional (2D) materials are intensively attractive for fabricating high sensitive photodetectors in terms of atomically thin flexible and ultrafast charge transport feature. Due to their atomically thin body, designing high performance detector requires new physical mechanisms and device structures. In this review, we classify design strategies and device structures into four categories depending on their physical mechanisms (photovoltaic effect, photoconductive effect, photothermoelectric effect or photobolometric effect, and surface plasma‐ wave‐assisted effect), and summarize the device performances. Finally, future prospects and development direction for 2D material photodetectors are described. Those design strategies descriptions about photoelectronic detector provide a reference for high responsivity and fast response speed photodetector at broadband sensing in the future.

Investigating PMMA as a potential insulator material for metal-insulator-metal diode

The metal-insulator-metal (MIM) diodes are considered very promising for high frequency applications because of its tunnelling-based ultrafast charge transport mechanism. A wide variety of materials are being explored in quest of developing these thin film devices. Recently, organic insulator-based MIM diodes have attracted significant attention to the researchers due to its several advantages and have become a promising alternative of conventional inorganic insulators. In the present work the feasibility of an organic polymer polymethyl methacrylate (PMMA) is investigated for insulating material of MIM junction. It is a transparent resin which is able to produce uniform and thin insulating layer with low defect density. In this work, an Al/PMMA/Cr structure is fabricated by optimising different process parameters in order to develop high performance, reproducible organic MIM diodes. The as fabricated device exhibited fairly asymmetric and nonlinear current-voltage characteristics, which reveals the viability of PMMA as candidate material for insulator layer in MIM diodes.

Ultrafast and nanoscale diodes

Charge carrier transport across interfaces of dissimilar materials (including vacuum) is the essence of all electronic devices. Ultrafast charge transport across a nanometre length scale is of fundamental importance in the miniaturization of vacuum and plasma electronics. With the combination of recent advances in electronics, photonics and nanotechnology, these miniature devices may integrate with solid-state platforms, achieving superior performance. This paper reviews recent modelling efforts on quantum tunnelling, ultrafast electron emission and transport, and electrical contact resistance. Unsolved problems and challenges in these areas are addressed.

Ultrafast Charge Transfer Pathways Through A Prototype Amino-Carboxylic Molecular Junction.

Charge transport properties of a vertically stacked organic heterojunction based on the amino-carboxylic (A-C) hydrogen bond coupling scheme are investigated by means of X-ray resonant photoemission and the core-hole clock method. We demonstrate that hydrogen bonding in molecular bilayers of benzoic acid/cysteamine (BA/CA) with an A-C coupling scheme opens a site selective pathway for ultrafast charge transport through the junction. Whereas charge transport from single BA layer directly coupled to the Au(111) is very fast and it is mediated by the phenyl group, the interposition of an anchoring layer of CA selectively hinders the delocalization of electrons from the BA phenyl group but opens a fast charge delocalization route through the BA orbitals close to the A-C bond. This evidences that hydrogen bonding established upon A-C recognition can be exploited to spatially/orbitally manipulate the charge transport properties of heteromolecular junctions.

Semimetallization of dielectrics in strong optical fields

At the heart of ever growing demands for faster signal processing is ultrafast charge transport and control by electromagnetic fields in semiconductors. Intense optical fields have opened fascinating avenues for new phenomena and applications in solids. Because the period of optical fields is on the order of a femtosecond, the current switching and its control by an optical field may pave a way to petahertz optoelectronic devices. Lately, a reversible semimetallization in fused silica on a femtosecond time scale by using a few-cycle strong field (~1 V/Å) is manifested. The strong Wannier-Stark localization and Zener-type tunneling were expected to drive this ultrafast semimetallization. Wider spread of this technology demands better understanding of whether the strong field behavior is universally similar for different dielectrics. Here we employ a carrier-envelope-phase stabilized, few-cycle strong optical field to drive the semimetallization in sapphire, calcium fluoride and quartz and to compare this phenomenon and show its remarkable similarity between them. The similarity in response of these materials, despite the distinguishable differences in their physical properties, suggests the universality of the physical picture explained by the localization of Wannier-Stark states. Our results may blaze a trail to PHz-rate optoelectronics.

Thermodynamic picture of ultrafast charge transport in graphene.

The outstanding charge transport properties of graphene enable numerous electronic applications of this remarkable material, many of which are expected to operate at ultrahigh speeds. In the regime of ultrafast, sub-picosecond electric fields, however, the very high conduction properties of graphene are not necessarily preserved, with the physical picture explaining this behaviour remaining unclear. Here we show that in graphene, the charge transport on an ultrafast timescale is determined by a simple thermodynamic balance maintained within the graphene electronic system acting as a thermalized electron gas. The energy of ultrafast electric fields applied to graphene is converted into the thermal energy of its entire charge carrier population, near-instantaneously raising the electronic temperature. The dynamic interplay between heating and cooling of the electron gas ultimately defines the ultrafast conductivity of graphene, which in a highly nonlinear manner depends on the dynamics and the strength of the applied electric fields.

Phonon-assisted ultrafast charge separation in the PCBM band structure

Organic solar cells must separate strongly bound electron-hole pairs into free charges. This is achieved at interfaces between electron donor and acceptor organic semiconductors. The most popular electron acceptor is the fullerene derivative PCBM. Electron-hole separation has been observed on femtosecond timescales, which is incompatible with conventional Marcus theories of organic transport. In this work we show that ultrafast charge transport in PCBM arises from its broad range of electronic eigenstates, provided by the presence of three closely spaced delocalised bands near the LUMO level. Vibrational fluctuations enable rapid transitions between these bands, which drives an electron transport of $\sim$3 nm within 100 fs. All this is demonstrated within a realistic tight binding Hamiltonian containing transfer integrals no larger than 8 meV.

Nano carbon conformal coating strategy for enhanced photoelectrochemical responses and long-term stability of ZnO quantum dots

A conformal coating strategy with nanocarbon to enhance photoelectrochemical responses and the long-term stability of ZnO quantum dots is described. Strong anchoring bonds between a ZnO core and nanocarbon shell ameliorate the poor electrochemical stability of ZnO (such as photocorrosion) in liquid electrolyte. The conjugation of the graphene QD and C60 to the ZnO QDs leads to 71% and 99% quenching of the UV photoluminescence (PL) emission, respectively. Also, the decay time of the nanocomposites at UV wavelengths measured much faster than that for the reference of bare ZnO QDs. The moderate energy states and good charge conductance of the nanocarbons result in ultrafast charge transport from the ZnO core to the nanocarbon shell. Thereby, the ZnO core–nanocarbon shell quantum dots shows significantly improved light harvesting performance. The PEC cell test for water oxidation and conventional degradation test using organic dyes exhibited that the photoelectrochemical activities could be significantly improved. At 1.23V (vs. RHE) in pH 6.9 electrolyte, 6 times enhanced photocurrent density was achieved by the conformal coating with C60 (0.235mA/cm2 for ZnO–C60 photoanodes). In particular, the strong Zn–O–C bond structures on the ZnO surface prevented photoinduced holes from being consumed by the photocorrosion reaction of ZnO, thereby improving long-term stability.

A model for ultra-fast charge transport in membrane proteins.

Isolated proteins have recently been observed to transport charge and reactivity over very long distances with extraordinary rates and near perfect efficiencies in spite of their site. This is not the case if the peptide is in water, where the efficiency of charge hopping to the next site is reduced to approximately 2%. Here, water is not an ideal solvent for charge transport. The issue at hand is how to explain such enormous charge transfer quenching in water compared to another typical medium, namely lipid. We performed molecular dynamics simulations to computationally substantiate the novel long-distance charge transfer yield of the polypeptides in lipids. This is characterized by the charge transfer persistent-distance decay constant and not by the rate, which is seldom, if ever, measured and hence not directly addressed here. This model can encompass an extremely wide range of yields over very long distances in peptides in various media. The calculations here demonstrate the good charge transport efficiency in lipids in contrast to the poor efficiency in water. The protein charge transport also exhibits a very strong anisotropic effect in lipids. The peptide secondary structure effect of charge transfer in membranes is analyzed in contrast to that in water. These results suggest that this model can be useful for the prediction of charge transfer efficiency in various environments of interest and indicate that the charge transfer is highly efficient in membrane proteins.

Sub-cycle control of terahertz high-harmonic generation by dynamical Bloch oscillations

Ultrafast charge transport in strongly biased semiconductors is at the heart of highspeed electronics, electro-optics, and fundamental solid-state physics. Intense light pulses in the terahertz (THz) spectral range have opened fascinating vistas: Since THz photon energies are far below typical electronic interband resonances, a stable electromagnetic waveform may serve as a precisely adjustable bias. Novel quantum phenomena have been anticipated for THz amplitudes reaching atomic field strengths. We exploit controlled THz waveforms with peak fields of 72 MV/cm to drive coherent interband polarization combined with dynamical Bloch oscillations in semiconducting gallium selenide. These dynamics entail the emission of phase-stable high-harmonic transients, covering the entire THz-to-visible spectral domain between 0.1 and 675 THz. Quantum interference of different ionization paths of accelerated charge carriers is controlled via the waveform of the driving field and explained by a quantum theory of inter- and intraband dynamics. Our results pave the way towards all-coherent THz-rate electronics.

Full-band Monte Carlo simulations of photo excitation in silicon diode structures

In this study, a full-band particle-based simulator is used to model photo-generated electron–hole pairs in Si to investigate ultra-fast charge transport. This work is motivated by previous simulations of transient transport in III–V material as well as recent experimental measurements of optically excited Si pin diodes. The simulation results show evidence of velocity overshoot at high fields (greater than 100 kV cm−1), and can be attributed to momentum relaxation.