Surface Charge Transfer Doping via Transition Metal Oxides for Efficient p-Type Doping of II-VI Nanostructures

ZnSe nanodisks:Ti3C2 MXenes-modified electrode for nucleic acid liquid biopsy with photoelectrochemical strategy

ZnSe nanodisks:Ti₃C₂ MXene complex was prepared for the first time. Based on its remarkable photoelectrochemical performance, combined with the enzyme-free toehold-mediated strand displacement reaction, a photoelectrochemical biosensor for the detection of the non-small-cell cancer biomarker ctDNA KRAS G12D was developed. ZnSe nanodisks were in situ grown on Ti₃C₂ MXene surface by two-step hydrothermal method. The high conductivity and adjustable band gap of MXene significantly enhanced the photoelectric response of ZnSe. Subsequently, the photoelectrochemical biosensor was prepared by combining with the signal amplification function of p-aminophenol and the enzyme-free toehold-mediated strand displacement reaction on the modified ITO electrode surface. Under the optimized conditions, the linear detection range is 0.5 ~ 100.0 fM, and the detection limit is 0.2 fM, which realizes the sensitive detection of KRAS G12D. The photoelectrochemical biosensor constructed opens up a new pathway for the preparation of new Mxene-based composite materials and the research of photoelectrochemical biosensor. Nucleic acid liquid biopsy with ZnSe nanodisks:Ti₃C₂ MXene photoelectroactive modified electrode.

Nanostructure-based photoelectrochemical sensing platforms for biomedical applications

As a newly developed and powerful analytical method, the use of photoelectrochemical (PEC) biosensors opens up new opportunities to provide wide applications in the early diagnosis of diseases, environmental monitoring and food safety detection. The properties of diverse photoactive materials are one of the essential factors, which can greatly impact the PEC performance. The continuous development of nanotechnology has injected new vitality into the field of PEC biosensors. In many studies, much effort on PEC sensing with semiconductor materials is highlighted. Thus, we propose a systematic introduction to the recent progress in nanostructure-based PEC biosensors to exploit more promising materials and advanced PEC technologies. This review briefly evaluates the several advanced photoactive nanomaterials in the PEC field with an emphasis on the charge separation and transfer mechanism over the past few years. In addition, we introduce the application and research progress of PEC sensors from the perspective of basic principles, and give a brief overview of the main advances in the versatile sensing pattern of nanostructure-based PEC platforms. This last section covers the aspects of future prospects and challenges in the nanostructure-based PEC analysis field.

Effect of electron-phonon interaction and valence band edge shift for carrier-type reversal in layered ZnS/rGO nanocomposites

The artificial stacking of nanohybrid films helps to enhance their properties and thus intrigues researchers to explore this possibility in emerging technologies. The layer-by-layer approach was used to fabricate samples of zinc sulfide/reduced graphene oxide (ZnS/rGO) by using spin coating technique. The structure and optoelectronic properties has been extensively studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV-VIS-NIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Hall measurements. Raman spectrum elucidates the phonon contribution of ZnS and breathing mode of κ-point phonons and sp² bonds of carbon atoms of rGO. The electron-phonon interactions reveal reduction in electron mobility and enhancement in holes contribution with rGO content leading to surface charge transfer doping (SCTD). XPS results explain the valence band edge and conduction band edge to form type-I band alignment to reconfirm carrier-type reversal. A change in the dispersion of refractive indices along with a small rise in the value of absorption coefficient in terahertz (THz) region for ZnS/rGO nanocomposite films has been observed. These results will open up new opportunities to furthering the science of this technologically important class of materials for future electronics.

Perovskite Core-Shell Nanowire Transistors: Interfacial Transfer Doping and Surface Passivation

While halide perovskite electronics are rapidly developing, they are greatly limited by the inferior charge transport and poor stability. In this work, effective surface charge transfer doping of vapor-liquid-solid (VLS)-grown single-crystalline cesium lead bromide perovskite (CsPbBr₃) nanowires (NWs) via molybdenum trioxide (MoO₃) surface functionalization is achieved. Once fabricated into NW devices, due to the efficient interfacial charge transfer and reduced impurity scattering, a 15× increase in the field-effect hole mobility (μ_h) from 1.5 to 23.3 cm²/(V s) is accomplished after depositing the 10 nm thick MoO₃ shell. This enhanced mobility is already better than any mobility value reported for perovskite field-effect transistors (FETs) to date. The photodetection performance of these CsPbBr₃/MoO₃ core-shell NWs is also investigated to yield a superior responsivity (R) up to 2.36 × 10³ A/W and an external quantum efficiency (EQE) of over 5.48 × 10⁵% toward the 532 nm regime. Importantly, the MoO₃ shell can provide excellent surface passivation to the CsPbBr₃ NW core that minimizes the diffusion of detrimental water and oxygen molecules, improving the air stability of CsPbBr₃/MoO₃ core-shell NW devices. All these findings evidently demonstrate the surface doping as an enabling technology to realize high-mobility and air-stable low-dimensional halide perovskite devices.

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.

Degenerately Doped Transition Metal Dichalcogenides as Ohmic Homojunction Contacts to Transition Metal Dichalcogenide Semiconductors

In search of an improved strategy to form low-resistance contacts to MoS₂ and related semiconducting transition metal dichalcogenides, we use ab initio density functional electronic structure calculations in order to determine the equilibrium geometry and electronic structure of MoO₃/MoS₂ and MoO₂/MoS₂ bilayers. Our results indicate that, besides a rigid band shift associated with charge transfer, the presence of molybdenum oxide modifies the electronic structure of MoS₂ very little. We find that the charge transfer in the bilayer provides a sufficient degree of hole doping to MoS₂, resulting in a highly transparent contact region.

Enhanced visible light absorption performance of SnS2 and SnSe2 via surface charge transfer doping

The layered two-dimensional (2D) SnS₂ and SnSe₂ have received intensive attention due to their sizable band gaps and potential properties. However, it has been shown that the visible light absorption of SnS₂ and SnSe₂ are restricted as photocatalysts and light-harvesting material absorbers for water splitting and high-performance optoelectronic devices. Herein, to enhance the visible light absorption performance of SnS₂ and SnSe₂, we performed a systematic investigation on tuning the electronic and optical properties of monolayers SnS₂ and SnSe₂ via surface charge transfer doping (SCTD) with the adsorption of molybdenum trioxide (MoO₃) and potassium (K) as surface dopants based on density functional theory. Our calculations reveal that MoO₃ molecules and K atoms can draw/donate electrons from/to SnS₂ and SnSe₂ as acceptors and donors, respectively. The adsorption of MoO₃ molecules introduces a new flat impurity state in the gap of the monolayers SnS₂/SnSe₂, and the Fermi level moves correspondingly to the top of valence band, resulting in a p-type doping of the monolayer SnS₂/SnSe₂. With the adsorption of K atoms, the electrons can transfer from K atoms to the monolayer of SnS₂ and SnSe₂, making K an effective electron-donating dopant. Meanwhile, the bandgaps of monolayers SnS₂ and SnSe₂ decrease after the MoO₃ and K doping, which leads to the appearance of appreciable new absorption peaks at around ∼650/480 and ∼600/680 nm, respectively, and yielding an enhanced visible light absorption of SnS₂ and SnSe₂. Our results unveil that SCTD is an effective way to improve the photocatalytic and light-harvesting performance of SnS₂ and SnSe₂, broadening their applications in splitting water and degrading environmental pollutants under sunlight irradiation.

Enhanced transport in transistor by tuning transition-metal oxide electronic states interfaced with diamond

High electron affinity transition-metal oxides (TMOs) have gained a central role in two-dimensional (2D) electronics by enabling unprecedented surface charge doping efficiency in numerous exotic 2D solid-state semiconductors. Among them, diamond-based 2D electronics are entering a new era by using TMOs as surface acceptors instead of previous molecular-like unstable acceptors. Similarly, surface-doped diamond with TMOs has recently yielded record sheet hole concentrations (2 × 10¹⁴ cm^-2) and launched the quest for its implementation in microelectronic devices. Regrettably, field-effect transistor operation based on this surface doping has been so far disappointing due to fundamental material obstacles such as (i) carrier scattering induced by nonhomogeneous morphology of TMO surface acceptor layer, (ii) stoichiometry changes caused by typical transistor fabrication process, and (iii) carrier transport loss due to electronic band energy misalignment. This work proposes and demonstrates a general strategy that synergistically surmounts these three barriers by developing an atomic layer deposition of a hydrogenated MoO₃ layer as a novel efficient surface charge acceptor for transistors. It shows high surface uniformity, enhanced immunity to harsh fabrication conditions, and benefits from tunable electronic gap states for improving carrier transfer at interfaces. These breakthroughs permit crucial integration of TMO surface doping into transistor fabrication flows and allow outperforming electronic devices to be reached.

Charge carrier-selective contacts for nanowire solar cells

Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends.

Balancing the carrier selectivity and extraction by the selective contacts is of vital importance to the performance of the nanowire solar cells. Here Oener et al. employ a permanent local gate to overcome this tradeoff and substantially increase the open-circuit voltage by 335 mV.

Ultrahigh Conductivity in Two-Dimensional InSe via Remote Doping at Room Temperature

Conductivity of two-dimenstional (2D) materials, which largely determines the efficiency and reliability of nanodevices, is proportional to the product of carrier concentration and mobility. Conventional doping, such as ionic substitution or introduction of vacancies, increases carrier concentration and decreases carrier mobility due to the scattering or trapping of carriers. We propose a remote-doping strategy that enables the simultaneous enhancement of both parameters. Density functional theory calculations in 2D InSe reveal that adsorbing the molecule tetrathiafulvalene (TTF) and applying a 4% external tensile strain leads to an increase in the carrier concentration of the TTF-InSe system that is 13 orders of magnitude higher than that of the pristine counterpart, whereas the carrier mobility is enhanced by 35% compared with the InSe monolayer. As a consequence of the synergetic role of molecule doping and strain engineering, ultrahigh conductivity of 1.85 × 10⁵ S/m is achieved in the TTF-InSe system at room temperature.

Atomically Thin p-n/p-n Nanodevices by Surface Charge Transfer Doping of Arsenene/Antimonene Heterostructures

Surface charge transfer doping (SCTD) is a promising technique to construct high-performance nanodevices because of its high reproducibility and high spatial selectivity and because it does little harm to the host semiconductor. Here, we performed a first-principles theoretical investigation to assess the effects of SCTD on the properties of two-dimensional (2D) arsenene, antimonene, and arsenene/antimonene van der Waals heterostructure as well. It was found that doping O or S on the surfaces of arsenene and antimonene could achieve efficient p-type doping, while doping Cs₂CO₃ on them could achieve n-type doping. Furthermore, when O and Cs₂CO₃ were co-doped on the two sides of the arsenene/antimonene heterostructure, a typical type-ii energy band alignment can be formed in O-arsenene/Cs₂CO₃-antimonene heterostructure, which effectively extends the range of the light absorption into the near-infrared region and facilitates the spatial separation of photogenerated electron-hole pairs. O- or S-doped arsenene and antimonene have tunable band gaps varying from 1.20 to 0.54 eV because of the doping-induced change of the conduction band minima (CBM), and Cs₂CO₃-doped arsenene and antimonene have band gaps of 2.02 and 1.36 eV, respectively, because of the changes of both valence band maxima and CBMs. This work offers a way to design p-n junctions with a tunable character, and the 2D p-n/p-n O-arsenene/Cs₂CO₃-antimonene heterostructure might be applied to electronic and optoelectronic nanodevices.

Controlled Electrodeposition of Photoelectrochemically Active Amorphous MoS x Cocatalyst on Sb2Se3 Photocathode

Amorphous molybdenum sulfide (a-MoS _x) is a promising hydrogen evolution catalyst owing to its low cost and high activity. A simple electrodeposition method (cyclic voltammetry) allows uniform formation of a-MoS _x films on conductive surfaces. However, the morphology of a-MoS _x deposited on a TiO₂/Sb₂Se₃ photocathode could be modulated by varying the starting potential. The cathodically initiated a-MoS _x showed conformal filmlike morphology, whereas anodic initiation induced inhomogeneous particulate deposition. The filmlike morphology of a-MoS _x was subjected to catalyst activation, which improved the photocurrent density and reduced the charge-transfer resistance at the semiconductor/electrolyte interface, as compared to that of its particulate counterpart. X-ray photoelectron spectroscopy confirmed that different chemical states of a-MoS _x (photoelectrochemically active sites) were developed on the basis of the electrodeposited a-MoS _x morphology. The research provides an effective approach for uniformly depositing cost-effective a-MoS _x on nanostructured photoelectrodes, for photoelectrochemical water splitting.

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.