Electrical Doping Research Articles

Electric Near-field Modulations of Charged Deoxyribonucleic Acid Nucleobases

Deoxyribonucleic acid (DNA) has been recently recognized as a promising material for nanophotonics due to its outstanding electro-optical tuning. Here using the realistic state-of-the-art quantum mechanical calculations, we carried out a systematic theoretical study on the electric near-field modulations of charged DNA nucleobases. Our results underline that electrical doping (the addition or removal of an electron) produces dramatic modulations to the electric near-field enhancements in the visible spectral range. Interestingly, electrical doping causes high-intensity electric near-field hotspot regions to emerge in the technologically relevant visible spectral range. Our results unveil electric near-field manipulation of DNA nucleobases, which might find applications in novel nanophotonic devices.

Recent advances in graphene and black phosphorus nonlinear plasmonics

Abstract Over the past decade, the plasmonics of graphene and black phosphorus (BP) were widely recognized as promising media for establishing linear and nonlinear light-matter interactions. Compared to the conventional metals, they support significant light-matter interaction of high efficiency and show undispersed optical properties. Furthermore, in contrast to the conventional metals, the plasmonic properties of graphene and BP structure can be tuned by electrical and chemical doping. In this review, a deep attention was paid toward the second- and third-order nonlinear plasmonic modes of graphene and BP. We present a theoretical framework for calculating the lifetime for surface plasmons modes of graphene and BP assisted by the coupled mode theory. The effect of the Fermi energy on the second-order and third-order nonlinear response is studied in detail. We survey the recent advances in nonlinear optics and the applications of graphene and BP-based tunable plasmonic devices such as light modulation devices, switches, biosensors, and other nonlinear photonic devices. Finally, we highlight a few representative current applications of graphene and BP to photonic and optoelectronic devices.

Approaching the Integer-Charge Transfer Regime in Molecularly Doped Oligothiophenes by Efficient Decarboxylative Cross-Coupling.

A library of symmetrical linear oligothiophene was prepared employing decarboxylative cross-coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross-coupling partners without the need of co-catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.

Approaching the Integer‐Charge Transfer Regime in Molecularly Doped Oligothiophenes by Efficient Decarboxylative Cross‐Coupling

AbstractA library of symmetrical linear oligothiophene was prepared employing decarboxylative cross‐coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross‐coupling partners without the need of co‐catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.

Metal-graphene hybrid active chiral metasurfaces for dynamic terahertz wavefront modulation and near field imaging

Chiral metasurfaces can achieve giant chiral optical responses and have been expanded from the optical band to other electromagnetic bands. Here, we propose a new method for dynamic terahertz circular dichroism (CD) manipulation in metasurfaces. By introducing a patterned and electrically doped graphene into the metamirror consists of double layer C-shaped split ring resonators (SRRs), efficient terahertz CD modulation is observed. Since the electrical doping of graphene changes the absorption loss of the metasuface cavity, the structure shows switching between a chiral metasurface and an ordinary metal mirror, which can be seen as "on" and "off" states. The calculation results show an efficient modulation of the terahertz CD in a large dynamic range. In addition, we also use the new method to design two metasurfaces for dynamic terahertz wavefront modulation and near-field digital imaging, both of which show a high-performance electrical switching. This method provides a new way for the design of active terahertz devices based on metasurfaces, and also promotes the applications of terahertz chiral metasurfaces in high-speed wireless communication and dynamic imaging.

Mutual electrical doping in polymers.

Two initially neutral conjugated semiconducting polymers are found to transfer electrons when put in contact in the solid state, leading to mutual electrical doping.

Charge-Transfer-Controlled Growth of Organic Semiconductor Crystals on Graphene.

Controlling the growth behavior of organic semiconductors (OSCs) is essential because it determines their optoelectronic properties. In order to accomplish this, graphene templates with electronic‐state tunability are used to affect the growth of OSCs by controlling the van der Waals interaction between OSC ad‐molecules and graphene. However, in many graphene‐molecule systems, the charge transfer between an ad‐molecule and a graphene template causes another important interaction. This charge‐transfer‐induced interaction is never considered in the growth scheme of OSCs. Here, the effects of charge transfer on the formation of graphene–OSC heterostructures are investigated, using fullerene (C60) as a model compound. By in situ electrical doping of a graphene template to suppress the charge transfer between C60 ad‐molecules and graphene, the layer‐by‐layer growth of a C60 film on graphene can be achieved. Under this condition, the graphene–C60 interface is free of Fermi‐level pinning; thus, barristors fabricated on the graphene–C60 interface show a nearly ideal Schottky–Mott limit with efficient modulation of the charge‐injection barrier. Moreover, the optimized C60 film exhibits a high field‐effect electron mobility of 2.5 cm2 V−1 s−1. These results provide an efficient route to engineering highly efficient optoelectronic graphene–OSC hybrid material applications.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Summary Capacitance-based techniques have been used to measure the electrical properties of halide perovskite solar cells (PSCs) such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. Here, we show that capacitance-based techniques cannot be used to reliably analyze the properties of defects in the perovskite layer or at its interface, because the high-frequency capacitance signature is due to the response of charge carriers in the hole-transport layer (HTL). For HTL-free PSCs, high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer because there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.

Organismic materials for beyond von Neumann machines

The elementary basis of intelligence in organisms with a central nervous system includes neurons and synapses and their complex interconnections forming neural circuits. In non-neural organisms such as slime mold with gel-like media, viscosity modulation enables adaptation to changing environments. At a larger scale, collective intelligence emerges via social interactions and feedback in animal colonies. Learning and memory are therefore multi-scale features that evolve as a result of constant interactions with the environment. There is growing interest in emulating such features of intelligence in computing machines and autonomous systems. Materials that can respond to their environment in a manner similar to organisms (referred to as “organismic materials”) therefore may be of interest as hardware components in artificial intelligence machines. In this brief review, we present a class of semiconductors called correlated oxides as candidates for learning machines. The term “correlated” refers to the fact that electrons in such lattices strongly interact and the ground state is not what is predicted by classical band theory. Such materials can undergo insulator–metal transitions at near ambient conditions under external stimuli such as thermal or electrical fields, strain, and chemical doping. Depending on the mechanism driving the transition, intermediate states can be metastable with different volatilities, and the time scales of phase change can be controlled over many orders of magnitude. The change in electronic properties can be sharp or gradual, leading to digital or analog behavior. These properties enable the realization of artificial neurons and synapses and emulate the associative and non-associative learning characteristics found in various organisms. We examine microscopic properties concerning electronic and structural transitions leading to collective behavior and theoretical treatments of the ground state and dynamical response, showcasing VO2 as a model system. Next, we briefly review algorithms designed from the plasticity demonstrated by phase changing systems. We conclude the brief review with suggestions for future research toward realizing non-von Neumann machines.

Circuit Level Modeling of Electrically Doped Adenine–Thymine Nanotube Based Field Effect Transistor

We investigate the gate-controlled, electrically doped tunnelling current in Adenine-Thymine heterojunction nanotube-based Field Effect Transistor (FET). This analytical model FET is designed by Density Functional Theory (DFT) and Non-Equilibrium Green's Function (NEGF) based First principle formalisms. It is demonstrated that Band to Band Tunnelling (BTBT) is possible in relaxed Adenine-Thymine heterostructure nanotube. The evaluation of BTBT tunnelling probability to estimate tunnelling current for only ±0.01V applied bias voltage is calculated using Wentzel-Kramers-Brillouin approximation. Electrical doping is introduced to eliminate the probability of fault generation. By keen observation on the shift of energy levels in the band structure, the availability of high transmission co-efficient peaks and current-voltage response we demonstrate the Schottky barrier nature for this geometrically pre-optimized bio-molecular FET. The doping concentration is varied from 0.0001V to 0.1V to achieve a substantially large amount of tunnelling current when the electronic temperature is kept at 300K. The E-k diagram or complex band structure of this heterostructure nanotube ensures its in-direct semi-conducting nature. This is a first attempt to present a circuit-level demonstration using this Adenine-Thymine nanotube-based bio-molecular FET and validate the obtained results with the existing approaches.

Graphene plasmonically induced analogue of tunable electromagnetically induced transparency without structurally or spatially asymmetry

Electromagnetically induced transparency (EIT) arises from the coherent coupling and interference between a superradiant (bright) mode in one resonator and a subradiant (dark) mode in an adjacent resonator. Generally, the two adjacent resonators are structurally or spatially asymmetric. Here, by numerical simulation, we demonstrate that tunable EIT can be induced by graphene ribbon pairs without structurally or spatially asymmetry. The mechanism originates from the fact that the resonate frequencies of the bright mode and the dark mode supported by the symmetrical graphene ribbon pairs can be respectively tuned by electrical doping levels, and when they are tuned to be equal the graphene plasmon coupling and interference occurs. The EIT in symmetrical nanostructure which avoids deliberately breaking the element symmetry in shape as well as in size facilitates the design and fabrication of the structure. In addition, the work regarding to EIT in the structurally symmetric could provide a fresh contribution to a more comprehensive physical understanding of Fano resonance.

Extended Wavelength Responsivity of a Germanium Photodetector Integrated With a Silicon Waveguide Exploiting the Indirect Transition

Photo-detection in the wavelength range 1850 to 2000 nm using evanescently-coupled germanium detectors grown on silicon waveguides is described. Devices were fabricated at a silicon photonics foundry using a process flow associated with operation in the O , C and L bands, and as such offer a solution for extended wavelength detection which is readily available. Intrinsic sensitivity is via indirect band transitions, which is enhanced by tensile strain and we postulate that it may be further enhanced by defects which arise from the thermal processes associated with Ge on Si growth. The responsivity of p-i-n detectors is 20 mA/W at 1850 nm falling to 5 mA/W at 2000 nm, for a detector length of 50 μm. Responsivity is suppressed by electrical doping in the germanium detector which provides parasitic absorption from free carriers. Modifications to the current design are suggested such that integrated germanium p-i-n detectors, directly grown on silicon waveguides would be suitable for high-bandwidth photo-detection up to at least a wavelength of 2000 nm. A Separate-Absorption-Charge-Multiplication Avalanche Photo-Detector is fabricated exploiting the same indirect transition. This detector has a responsivity of 0.31 A/W at 1850 nm and 0.08 A/W at 1970 nm, for a detector length of only 14 μm.

Performance Enhancement of Ternary Polymer Solar Cells Induced by Tetrafluorotetracyanoquinodimethane Doping

Molecular electrical doping has been proved as an effective strategy to improve the performance of traditional binary polymer solar cells (PSCs). However, its effectiveness is not demonstrated yet ...

N-Type Doping of Organic Semiconductors: Immobilization via Covalent Anchoring

Electrical doping is an important tool in the design of organic devices to modify charge carrier concentration in and Fermi level position of organic layers. The undesired diffusion of dopant molec...

(Invited) A Simple and Efficient Solution-Based Technique to Electrically Dope Organic Semiconductors

The printed electronics industry offers a paradigm change in manufacturing, cost and environmental impact when compared to the conventional semiconductor industry. Printed electronic devices are expected to be mass-produced from less-energy-demanding processes over large areas and on flexible substrates, with techniques that closely resemble the well-known mass production of printed media on paper. Still, critical to the widespread of these devices is the ability to convert lab produced “champion” figures into reliable industrial-scale products. The well-established strategies for the design and fabrication of efficient lab-scale printed electronics are not always compatible with large-area manufacturing, where several additional requirements must be met. Solution-based electrical doping of organic semiconductors has received increased attention as a technique with the potential to comply with some of these additional requirements. Stable ink-type formulations of dopant, semiconductor and solvents have been reported, some of which may be compatible with roll-to-roll coating. Yet, the ability to fabricate vertical doping gradients, critical to maintain optimal optoelectronic performance at reduced cost, has proven challenging. Vertical doping gradients would reduce the amount of coating steps without a loss in optoelectronic device functionality, simplifying manufacturing and preventing defects to propagate. In 2017 we reported a simple technique to fabricate vertical electrical p-type doping gradients in organic semiconductor films by dipping them in solutions. Still, the initial approach, which involved 12-molybdophosphoric acid (‘PMA’ dopant) and nitromethane (processing solvent), was highly solvent selective, and the use of nitromethane was found limiting. Thus, in 2018, our team replaced nitromethane with acetonitrile and showed that the PMA-acetonitrile solution became stable in air without losing its doping ability. Furthermore, organic solar cells fabricated using the updated approach showed longer air stability. In this talk we will start by discussing the basics of the solution-based technique to electrically p-type organic semiconductors using PMA, and compare it against similar, well-studied approaches. Then, we will emphasize on our latest insights on the path to unravel, from a molecular perspective, how organic semiconductors are doped down to a limited depth from solution, using PMA molecules. That is, what is the interplay between the organic film and PMA molecules in solution that leads to p-type electrical doping of the semiconducting layer? Additionally, what are the key parameters that determine the efficiency and extent of the process?

Electronic transport properties of electrically doped cytosine‐based optical molecular switch with single‐wall carbon nanotube electrodes

This study represents an empirical model of cytosine-based optical molecular switch. This possible biomolecular switch has been designed using the first principle approach which is based on density functional theory and non-equilibrium Green's function. The quantum-ballistic transport property and current-voltage (I-V) characteristics of cytosine-based optomolecular switch have been investigated at 25 THz operating frequency. The influence of highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps on the electronic transmission and I-V characteristics has been discussed in detail. The aim of this study is to highlight the minimum conformational change during a single ON-OFF switching cycle. The biomolecule comprises switching behaviour when converts from straightened to twisted form during photo-excitement. The straightened and twisted forms of the molecule are represented as logic `0' and logic `1', respectively. This p and n regions of this switch has been made using electrical doping process. The current through the twisted form of the cytosine biomolecule is ~1000 times higher than the straightened form. The maximum switching ratio 62.1 is obtained at 1 V bias. The origin of the switching behaviour of the biomolecule can be interpreted by quantum-ballistic transport model along with HOMO-LUMO gaps.

Reconfigurable edge-state engineering in graphene using LaAlO3/SrTiO3 nanostructures

The properties of graphene depend sensitively on doping with respect to the charge-neutrality point (CNP). Tuning the CNP usually requires electrical gating or chemical doping. Here, we describe a technique to reversibly control the CNP in graphene with nanoscale precision, utilizing LaAlO3/SrTiO3 (LAO/STO) heterostructures and conductive atomic force microscope (c-AFM) lithography. The local electron density and resulting conductivity of the LAO/STO interface can be patterned with a conductive AFM tip [Cen et al., Nat. Mater. 7, 298 (2008)] and placed within two nanometers of an active graphene device [Huang et al., APL Mater. 3, 062502 (2015)]. The proximal LAO/STO nanostructures shift the position of graphene CNP by ∼1012 cm−2 and are also gateable. Here, we use this effect to create reconfigurable edge states in graphene, which are probed using the quantum Hall effect. Quantized resistance plateaus at h/e2 and h/3e2 are observed in a split Hall device, demonstrating edge transport along the c-AFM written edge that depends on the polarity of both the magnetic field and direction of currents. This technique can be readily extended to other device geometries.

Recent Progress on Graphene-Functionalized Metasurfaces for Tunable Phase and Polarization Control.

The combination of graphene and a metasurface holds great promise for dynamic manipulation of the electromagnetic wave from low terahertz to mid-infrared. The optical response of graphene is significantly enhanced by the highly-localized fields in the meta-atoms, and the characteristics of meta-atoms can in turn be modulated in a large dynamic range through electrical doping of graphene. Graphene metasurfaces are initially focused on intensity modulation as modulators and tunable absorbers. In this paper, we review the recent progress of graphene metasurfaces for active control of the phase and the polarization. The related applications involve, but are not limited to lenses with tunable intensity or focal length, dynamic beam scanning, wave plates with tunable frequency, switchable polarizers, and real-time generation of an arbitrary polarization state, all by tuning the gate voltage of graphene. The review is concluded with a discussion of the existing challenges and the personal perspective of future directions.

Quantitative scanning spreading resistance microscopy on n-type dopant diffusion profiles in germanium and the origin of dopant deactivation

Diffusion profiles of arsenic and antimony in undoped and carbon doped germanium (Ge), respectively, were analysed by means of scanning spreading resistance microscopy (SSRM). Whereas earlier secondary ion mass spectrometry analyses have determined the distribution of the chemical concentration of dopants and carbon, the electrically active defect concentration is quantified by SSRM using appropriate calibration samples and a preparation technique that reduces the surface roughness and its density of electronic states. Pronounced differences between the chemical and electrical dopant profiles are observed and consistently described by the formation of inactive dopant defect complexes in the framework of the vacancy mediated diffusion of donor atoms in Ge. This reveals that donor deactivation occurs during dopant diffusion at elevated temperatures.

Nonaggregating Doped Polymers Based on Poly(3,4-Propylenedioxythiophene)

Electrical doping of organic semiconductors is critical for their use in electrical devices. However, many high-performance semiconductors exhibit drastically reduced solubilities when doped in solution, making it difficult to deconvolute the roles of doping and morphological changes on their electrical properties. Here, we report the synthesis of a semiconducting polymer based on poly(3,4-propylenedioxythiophene) (ProDOT) substituted with oligo(ethylene glycol) (EG) side chains that is designed to solvate dopant molecules. When doped with F4TCNQ in solution, the polymer P(ProDOT-EG) undergoes efficient charge transfer while remaining soluble with no indication of aggregation. The electrical conductivity of thin films cast from heavily doped solutions is ∼1 S/cm; optical spectroscopy reveals that the doping efficiency is reduced upon formation of the solid film. Diffusion of F4TCNQ from the vapor phase into films of neutral P(ProDOT-EG) yields comparable conductivities to films cast from doped solutions. ...