Surface Charge Transfer Doping of Monolayer Phosphorene via Molecular Adsorption

Semiconductivity Transition in Silicon Nanowires by Hole Transport Layer

The surface of nanowires is a source of interest mainly for electrical prospects. Thus, different surface chemical treatments were carried out to develop recipes to control the surface effect. In this work, we succeed in shifting and tuning the semiconductivity of a Si nanowire-based device from n- to p-type. This was accomplished by generating a hole transport layer at the surface by using an electrochemical reaction-based nonequilibrium position to enhance the impact of the surface charge transfer. This was completed by applying different annealing pulses at low temperature (below 400 °C) to reserve the hydrogen bonds at the surface. After each annealing pulse, the surface was characterized by XPS, Kelvin probe measurements, and conductivity measured by FET based on a single Si NW. The mechanism and conclusion were supported experimentally and theoretically. To this end, this strategy has been demonstrated as an essential tool which could pave a new road for regulating semiconductivity and for other low-dimensional nanomaterials.

Ambipolar transport in van der Waals black arsenic field effect transistors

Black arsenic (BAs) is an elemental van der Waals semiconductor that is promising for a wide range of electronic and photonic applications. The narrow bandgap and symmetric band structure suggest that ambipolar (both n- and p-type) transport should be observable, however, only p-type transport has been experimentally studied to date. Here, we demonstrate and characterize ambipolar transport in exfoliated BAs field effect transistors. In the thickest flakes (∼ 80 nm), maximum currents, I _max, up to 60 μA μm^-1 and 90 μA μm^-1are achieved for hole and electron conduction, respectively. Room-temperature hole (electron) mobilities up to 150 cm² V^-1 s^-1 (175 cm² V^-1 s^-1) were obtained, with temperature-dependence consistent with a phonon-scattering mechanism. The Schottky barrier height for Ni contacts to BAs was also extracted from the temperature-dependent measurements. I _max for both n- and p-type conductivity was found to decrease with reduced thickness, while the ratio of I _max to the minimum current, I _min, increased. In the thinnest flakes (∼ 1.5 nm), only p-type conductivity was observed with the lowest value of I _min = 400 fA μm^-1. I _max/I _min ratios as high as 5 × 10⁵ (5 × 10²) were obtained, for p- (n-channel) devices. Finally, the ambipolarity was used to demonstrate a complementary logic inverter and a frequency doubling circuit.

Modulating the Electronic, Optical, and Transport Properties of CdTe and ZnTe Nanostructures with Organic Molecules: A Theoretical Investigation

In this paper, we systematically investigated the electronic, optical, and transport properties of CdTe and ZnTe nanostructures before and after adsorption with benzyl viologen (BV) and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) organic molecules based on the first principles calculation. First, the band gaps of CdTe and ZnTe nanostructures obviously decrease after BV and F4-TCNQ adsorptions. Interestingly, the electronic property calculation shows that BV and F4-TCNQ can donate/accept electrons to/from the surface of CdTe and ZnTe nanostructures, leading to an effective n-/p-type doping, respectively. Second, the optical absorption in a broad spectral range (from visible to near-infrared) of CdTe and ZnTe is significantly improved by adsorption of BV and F4-TCNQ molecules, offering great opportunities for the use of CdTe and ZnTe nanostructures in renewable energy fields. Lastly, the electrical transfer characteristics on CdTe and ZnTe nanostructure-based field-effect transistors clearly showed that the conduction of the nanostructures can be rationally tuned into n- and p-type conductivity with BV and F4-TCNQ adsorptions, respectively. Our work clearly demonstrates that the electronic, optical, and transport properties of CdTe and ZnTe nanostructures are effectively modulated by adsorption of BV and F4-TCNQ, which can be used to construct high-performance electronic and optoelectronic devices.

Prospects for Functionalizing Elemental 2D Pnictogens: A Study of Molecular Models

Despite the intense amount of attention and huge potential of 2D-layered pnictogens for applications in chemistry, physics, and materials science, there has yet to be a robust strategy developed to systematically functionalize them to tailor their properties. This is due to a number of factors, including practical instability toward ambient conditions, difficulty in characterizing modified materials, and also more inherent reactivity issues. Here, avenues for functionalization are discussed using examples of molecular models from the wider literature, along with their possible advantages and likely pitfalls. Finally, a critical appraisal of the current field and its future is offered.

Theoretical study of a p-n homojunction SiGe field-effect transistor via covalent functionalization

p-n homojunctions are superior to p-n heterojunctions in constructing nanoscale functional devices, owing to the excellent crystallographic alignment. We tune the electronic properties of monolayer siligene (SiGe) into p/n-type via the covalent functionalization of electrophilic/nucleophilic dopants, using ab initio quantum transport calculations. It is found that the n-type doping effect of K atoms is stronger than that of benzyl viologen (BV) molecule on the surface of SiGe monolayer, owing to the strong covalent interaction. Both of p-type 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-adsorbed and n-type 4 K-adsorbed SiGe systems show enhanced optical absorption in the infrared region, indicating their promising applications in infrared optoelectronic devices. By spatially adsorbing F4TCNQ molecule and K atoms on the source and drain leads, respectively, we designed a p-n homojunction SiGe field-effect transistor (FET). It is predicted that the built F4TCNQ-4K/SiGe FET can meet the requirements for high-performance (the high current density) and low-power (low subthreshold swing (SS)) applications, according to the International Technology Roadmap for Semiconductors in 2028. The present study gains some key insights into the importance of surface functionalization in constructing p-n homojunction electronic and optoelectronic devices based on monolayer SiGe.

Molecular doping of blue phosphorene: a first-principles investigation

Using first-principles calculations, we show that p-doped blue phosphorene can be obtained by molecular doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) and 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F₆-TNAP), whereas n-doped blue phosphorene can be realized by doping with tetrathiafulvalene (TTF) and cyclooctadecanonaene (CCO). Moreover, the doping gap can be effectively modulated in each case by applying an external perpendicular electric field. The optical absorption of blue phosphorene can be considerably enhanced in a broad spectral range through the adsorption of CCO, F₄-TCNQ, and F₆-TNAP molecules, suggesting potential of the doped materials in the field of renewable energy.

Property-Activity Relationship of Black Phosphorus at the Nano-Bio Interface: From Molecules to Organisms

As a novel member of the two-dimensional nanomaterial family, mono- or few-layer black phosphorus (BP) with direct bandgap and high charge carrier mobility is promising in many applications such as microelectronic devices, photoelectronic devices, energy technologies, and catalysis agents. Due to its benign elemental composition (phosphorus), large surface area, electronic/photonic performances, and chemical/biological activities, BP has also demonstrated a great potential in biomedical applications including biosensing, photothermal/photodynamic therapies, controlled drug releases, and antibacterial uses. The nature of the BP-bio interface is comprised of dynamic contacts between nanomaterials (NMs) and biological systems, where BP and the biological system interact. The physicochemical interactions at the nano-bio interface play a critical role in the biological effects of NMs. In this review, we discuss the interface in the context of BP as a nanomaterial and its unique physicochemical properties that may affect its biological effects. Herein, we comprehensively reviewed the recent studies on the interactions between BP and biomolecules, cells, and animals and summarized various cellular responses, inflammatory/immunological effects, as well as other biological outcomes of BP depending on its own physical properties, exposure routes, and biodistribution. In addition, we also discussed the environmental behaviors and potential risks on environmental organisms of BP. Based on accumulating knowledge on the BP-bio interfaces, this review also summarizes various safer-by-design strategies to change the physicochemical properties including chemical stability and nano-bio interactions, which are critical in tuning the biological behaviors of BP. The better understanding of the biological activity of BP at BP-bio interfaces and corresponding methods to overcome the challenges would promote its future exploration in terms of bringing this new nanomaterial to practical applications.

Two-Dimensional Pnictogen for Field-Effect Transistors

Two-dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group-VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next-generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field-effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.

Ballistic transport simulation of acceptor-donor C3N/C3B double-wall hetero-nanotube field effect transistors

The one-dimensional (1D) acceptor-donor (A-D) hetero-nanotube (HNT) has attracted much attention as a potential candidate for a channel structure of next-generation field effect transistors (FETs). Herein, we designed A-D C₃N/C₃B double-wall (dw) HNTs by coaxially wrapping C₃N and C₃B nanotubes together. By using density functional theory (DFT) combined with the nonequilibrium Green's function (NEGF) formalism, we studied the electronic properties and ballistic transport behavior of C₃N/C₃B dwHNTs in comparison with the ones in vertical stacking contact. The remarkable charge transfer between C₃N and C₃B nanotubes and band hybridization in C₃N/C₃B dwHNTs originate from the strong intertubular π-π stacking interaction. In the simulated all-around-gated FET devices, (12,0) C₃N/(6,0) C₃B dwHNT with 2 repeated wrapping units possesses the optimal performance, including rectifying ratio and subthreshold swing (SS), by enhancing the A-D asymmetry. Our work suggests that coaxial wrapping is a method superior to vertical stacking in constructing A-D HNTs for high-performance electronic and optoelectronic devices based on 1D materials.

Electric-field-tunable molecular adsorption on germanane

Fully-hydrogenated germanene, named germanane, represents a new nanostructured material for a variety of potential applications, such as electronics and optoelectronics. However, a critical requirement for developing practical and reliable electronic devices based on germanane consists of achieving a flexibly controllable charge carrier and doping level. Different to the conventional doping methods such as ion implantation and diffusion, by first-principles calculations we demonstrate that tetracyanobenzene (TCNB) molecular adsorption could introduce effective p-type doping in germanane due to the combination of germanane with electroactive acceptor molecule TCNB. The corresponding energy difference between the empty band minimum of the dopant and the valence band maximum for electron excitation is 0.173 eV. More importantly, this nondestructive p-type doping could be linearly tuned under an external E-field. Analysis of charge transfer by means of the equivalent capacitor model and the shift of energy levels in the superstructure of germanane/TCNB further reveals that the superposition of the external E-field and molecular adsorption-induced internal E-field plays a key role in the charge transfer between TCNB and germanane, especially in achieving a controllable p-type molecular doping level in germanane. Such convenient and flexible E-field-engineering of p-type molecular doping in germanane would be very helpful for potential applications of germanane-based electronic and optoelectronic devices in the future.

Noncovalent Functionalization of 2D Black Phosphorus with Fluorescent Boronic Derivatives of Pyrene for Probing and Modulating the Interaction with Molecular Oxygen

We studied the chemical-physical nature of interactions involved in the formation of adducts of two-dimensional black phosphorus (2D BP) with organoboron derivatives of a conjugated fluorescent molecule (pyrene). Time-resolved fluorescence spectroscopy showed a stabilization effect of 2D BP on all derivatives, in particular for the adducts endowed with the boronic functionalities. Also, a stronger modulation of the fluorescence decay with oxygen was registered for one of the adducts compared to the corresponding organoboron derivative alone. Nuclear magnetic resonance experiments in suspension and density functional theory simulations confirmed that only noncovalent interactions were involved in the formation of the adducts. The energetic gain in their formation arises from the interaction of P atoms with both C atoms of the pyrene core and the B atom of the boronic functionalities, with a stronger contribution from the ester with respect to the acid one. The interaction results in the lowering of the band gap of 2D BP by around 0.10 eV. Furthermore, we demonstrated through Raman spectroscopy an increased stability toward oxidation in air of 2D BP in the adducts in the solid state (more than 6 months). The modification of the electronic structure at the interface between 2D BP and a conjugated organic molecule through noncovalent stabilizing interactions mediated by the B atom is particularly appealing in view of creating heterojunctions for optoelectronic, photonic, and chemical sensing applications.

Modifying the sensibility of nonmetal-doped phosphorene by local or global properties

The dopant atom X can modify the sensibility of X-doped phosphorene by tuning the electronic properties of X-doped phosphorene surfaces effectively. According to the adsorption strength and the amount of charge transfer between the adsorption species and X-doped phosphorene surfaces, the adsorption species can be roughly divided into three types.

The organic-2D transition metal dichalcogenide heterointerface

Since the first isolation of graphene, new classes of two-dimensional (2D) materials have offered fascinating platforms for fundamental science and technology explorations at the nanometer scale. In particular, 2D transition metal dichalcogenides (TMD) such as MoS2 and WSe2 have been intensely investigated due to their unique electronic and optical properties, including tunable optical bandgaps, direct-indirect bandgap crossover, strong spin-orbit coupling, etc., for next-generation flexible nanoelectronics and nanophotonics applications. On the other hand, organics have always been excellent materials for flexible electronics. A plethora of organic molecules, including donors, acceptors, and photosensitive molecules, can be synthesized using low cost and scalable procedures. Marrying the fields of organics and 2D TMDs will bring benefits that are not present in either material alone, enabling even better, multifunctional flexible devices. Central to the realization of such devices is a fundamental understanding of the organic-2D TMD interface. Here, we review the organic-2D TMD interface from both chemical and physical perspectives. We discuss the current understanding of the interfacial interactions between the organic layers and the TMDs, as well as the energy level alignment at the interface, focusing in particular on surface charge transfer and electronic screening effects. Applications from the literature are discussed, especially in optoelectronics and p-n hetero- and homo-junctions. We conclude with an outlook on future scientific and device developments based on organic-2D TMD heterointerfaces.

Tuning the Carrier Confinement in GeS/Phosphorene van der Waals Heterostructures

Van der Waals (vdW) heterostructures, which have the advantage of integrating excellent properties of the stacked 2D materials by vdW interactions, have gained increasing attention recently. In this work, within the framework of density functional theory calculations, the electronic properties of vdW heterostructure composed of phosphorene (BP) in black phosphorus phase and GeS monolayer are systematically explored. The results show that the carriers are not separated for both lattice-match and lattice-mismatch heterostructures. For the lattice-match heterostructure, it is found that changing monolayer of GeS to bilayer can increase the energy difference of valence band offsets between GeS and BP, thus realizing electron-hole separation. For the lattice-mismatch heterostructure, altering the layer distance can transform the heterostructure into a typical type-I alignment, but applying the electric field or doping with 2, 3, 5, 6-tetrafluoro-7, 7, 8, 8-tetracyanoquinodimethane (F4TCNQ) can make it display a perfect desirable type-II alignment, where holes migration and electrons transfer are revealed to account respectively for the phenomenon of carrier separation. It is believed that the work would greatly enlarge the potential application of the BP-based heterostructures in photoelectronics and further stimulate the investigation enthusiasms on other fashionable heterostructures and even unassuming heterostructures in which the charming electronic properties can be modulated to emerge by various general methods.

Recovery Mechanism of Degraded Black Phosphorus Field-Effect Transistors by 1,2-Ethanedithiol Chemistry and Extended Device Stability

Black phosphorus (BP) has drawn enormous attention for both intriguing material characteristics and electronic and optoelectronic applications. In spite of excellent advantages for semiconductor device applications, the performance of BP devices is hampered by the formation of phosphorus oxide on the BP surface under ambient conditions. It is thus necessary to resolve the oxygen-induced degradation on the surface of BP to recover the characteristics and stability of the devices. To solve this problem, it is demonstrated that a 1,2-ethanedithiol (EDT) treatment is a simple and effective way to remove the bubbles formed on the BP surface. The device characteristics of the degraded BP field-effect transistor (FET) are completely recovered to the level of the pristine cases by the EDT treatment. The underlying principle of bubble elimination on the BP surface by the EDT treatment is systematically analyzed by density functional theory calculation, atomic force microscopy, and X-ray photoelectron spectroscopy analysis. In addition, the performance of the hexagonal boron nitride-protected BP FET is completely retained without changing device characteristics even when exposed to 30 d or more in air. The EDT-induced recovering effect will allow a new route for the optimization of electronic and optoelectronic devices based on BP.

The improvement of photocatalytic activity of monolayer g-C3N4via surface charge transfer doping

Graphite-like carbon nitride (g-C₃N₄) has attracted much attention due to its peculiar photocatalytic performance as a visible-light-responsive photocatalyst. However, its insufficient sunlight absorption is not conducive to the photocatalytic activity of the g-C₃N₄. Herein, by using first-principles density functional theory (DFT) calculations, we demonstrated a simple yet efficient way to achieve improvement of photocatalytic activity of monolayer g-C₃N₄via surface charge transfer doping (SCTD) using the electron-drawing tetracyanoquinodimethane (TCNQ) and electron-donating tetrathiafulvalene (TTF) as surface dopants. Our calculations revealed that the electronic properties of monolayer g-C₃N₄ can be affected by surface modification with TCNQ and TTF. These dopants are capable of drawing/donating electrons from/to monolayer g-C₃N₄, leading to the accumulation of holes/electrons injected into the monolayer g-C₃N₄. Correspondingly, the Fermi levels of monolayer g-C₃N₄ were shifted towards the valence/conduction band regions after surface modifications with TCNQ and TTF, along with the increase/decrease of work functions. Moreover, the optical property calculations demonstrated that the TCNQ and TTF modifications could significantly broaden the optical absorption of monolayer g-C₃N₄ in the visible-light regions, yielding an improvement in the photocatalytic activity of monolayer g-C₃N₄. Our results unveil that SCTD is an effective way to tune the electronic and optical properties of monolayer g-C₃N₄, thus improving its photocatalytic activity and broadening its applications in splitting water and degrading environmental pollutants under sunlight irradiation.

The improvement of optical adsorption for monolayer g-C₃N₄via surface charge transfer doping.

Novel electronic structures and enhanced optical properties of boron phosphide/blue phosphorene and F4TCNQ/blue phosphorene heterostructures: a DFT + NEGF study

Blue phosphorene (Blue-p), an allotrope of black phosphorene, has attracted extensive interest due to its hexagonal crystal with a flat arranged layer of phosphorus atoms.

Doping Phosphorene with Holes and Electrons through Molecular Charge Transfer

An important aspect of phosphorene, the novel two-dimensional semiconductor, is whether holes and electrons can both be doped in this material. Some reports found that only electrons can be preferentially doped into phosphorene. There are some theoretical calculations showing charge-transfer interaction with both tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE). We have carried out an investigation of chemical doping of phosphorene by a variety of electron donor and acceptor molecules, employing both experiment and theory, Raman scattering being a crucial aspect of the study. We find that both electron acceptors and donors interact with phosphorene by charge-transfer, with the acceptors having more marked effects. All the three Raman bands of phosphorene soften and exhibit band broadening on interaction with both donor and acceptor molecules. First-principles calculations establish the occurrence of charge-transfer between phosphorene with donors as well as acceptors. The absence of electron-hole asymmetry is noteworthy.

Surface Charge Transfer Doping of Low-Dimensional Nanostructures toward High-Performance Nanodevices

Device applications of low-dimensional semiconductor nanostructures rely on the ability to rationally tune their electronic properties. However, the conventional doping method by introducing impurities into the nanostructures suffers from the low efficiency, poor reliability, and damage to the host lattices. Alternatively, surface charge transfer doping (SCTD) is emerging as a simple yet efficient technique to achieve reliable doping in a nondestructive manner, which can modulate the carrier concentration by injecting or extracting the carrier charges between the surface dopant and semiconductor due to the work-function difference. SCTD is particularly useful for low-dimensional nanostructures that possess high surface area and single-crystalline structure. The high reproducibility, as well as the high spatial selectivity, makes SCTD a promising technique to construct high-performance nanodevices based on low-dimensional nanostructures. Here, recent advances of SCTD are summarized systematically and critically, focusing on its potential applications in one- and two-dimensional nanostructures. Mechanisms as well as characterization techniques for the surface charge transfer are analyzed. We also highlight the progress in the construction of novel nanoelectronic and nano-optoelectronic devices via SCTD. Finally, the challenges and future research opportunities of the SCTD method are prospected.

Novel polystyrene-based nanocomposites by phosphorene dispersion

Polystyrene-based phosphorene nanocomposites were prepared by a solvent blending procedure allowing the embedding of black phosphorus (BP) nanoflakes in the polymer matrix.