Metal-semiconductor Heterojunction Research Articles

Electrical contact property and control effects for stable T(H)-TaS2/C3B metal-semiconductor heterojunctions.

Metal-semiconductor heterojunctions are the basis for developing new electronic devices. Here, T(H)-TaS2/C3B metal-semiconductor heterostructures are constructed by different phase T- and H-TaS2 monolayers combined with the C3B monolayer. The calculated corrected binding energies, phonon band structures, elastic constants, and molecular dynamics simulations indicated that both heterojunctions are highly stable, meaning that T(H)-TaS2/C3B heterojunctions possibly exist in experiments. The electronic property calculations showed that the intrinsic T(H)-TaS2/C3B heterojunction is an n(p)-type Schottky contact with a low Schottky barrier height (SBH), which is very important for the design of high-performance field-effect transistors. The electronic properties of the T(H)-TaS2/C3B heterojunctions can be controlled by varying the vertical strain and external electric field; however, the strain only resulted in a small change in the heterojunction SBH. Nevertheless, under external electrical field control, the T-TaS2/C3B heterojunction could manage a transition from an n-type Schottky contact to an n-type Ohmic contact and the H-TaS2/C3B heterojunction could be altered from a p-type Schottky contact to a p-type Ohmic contact. These findings provide theoretical insights into the electronic and electrical contact properties of the T(H)-TaS2/C3B heterojunction, which could be beneficial for developing n-type MOS and p-type MOS transistors.

An Electron-Coupled Co-ZrO2 Nanodot Heterojunction Electrocatalyst with Lewis Acid-Base Site Pairs Enables High Redox Reaction Kinetics of Li-S Batteries.

Using electrocatalysts is effective in solving the slow reaction kinetics of polysulfides in Li-S batteries, but designing stable electrocatalysts with an integrated adsorption-catalysis-desorption system is challenging. Here, we report a stable metal-semiconductor (Co-ZrO2) heterojunction electrocatalyst fabricated by assembling electron-coupled Co-ZrO2 nanodots into macroporous carbon nanofibers. The Co-ZrO2 contact causes interfacial electron enrichment and electron transfer from Co to ZrO2, which creates abundant Lewis-acid sites on Co that can adsorb polysulfides. Simultaneously, the enriched interfacial electrons can activate the S-S bond and boost the catalytic conversion of long-chain polysulfides, while the ZrO2 with Lewis-base sites facilitate the desorption of short-chain polysulfides from the electrocatalyst. Moreover, the nanodot heterojunctions show great chemical stability and high redox reaction kinetics of polysulfides. Li-S batteries show high discharge capacities of 954.5 mA h·g-1 at 0.5 C with a retention of 84.9% over 200 cycles, and 710.2 mA hg-1 at 1 C with a retention of 98.6% over 200 cycles. This study provides an effective strategy for developing active and durable electrocatalysts for Li-S batteries.

Wastewater Treatment Using High-Performance In Situ Formed Double-Heterojunction Janus Photocatalyst Microparticles Shaped via a Microfluidic Device.

In this work, a heterogeneous photocatalysis system is fabricated for treating wastewater containing organic dyes and pharmaceutical substances. Double-heterojunction Janus photocatalysts are formed on the surface of size-tunable polydimethylsiloxane (PDMS) microparticles shaped via simple and low-cost coflow microfluidic devices. Ag0/Ag0-TiO2/TiO2 Janus-like photocatalysts are synthesized on the surface of porous PDMS microparticles as the support in which the metal-semiconductor heterojunction of Ag0/Ag0-TiO2 and the second heterojunction of Ag0-TiO2/TiO2 are created in situ, leading to the formation of Ag0/Ag0-TiO2/TiO2@PDMS photocatalysis systems. To form the heterojunctions on the PDMS surface, the polymer chain etching method is employed as a desired strategy to have half of the TiO2 nanoparticles on the surface of microparticles, which are treated by a Ag source. Using salt additives and the etching method, PDMS microparticles are made porous, providing more surface area for photoreactions. Surprisingly, the highest decomposition efficiencies of 94.4 and 91.1% are achieved for rhodamine B(RhB) and tetracycline (TC), respectively, under visible light for 60 min pH 11, a light source at a distance of 2 cm, 5 mM AgNO3, 10 wt % TiO2, 7 wt % NaCl, and 20 gm/L photocatalyst, which are conditions that result in the best performance for RhB degradation. Regarding the stability of the photocatalysts, no significant change is observed in the performance after five cycles.

An Atomically Resolved Schottky Barrier Height Approach for Bridging the Gap between Theory and Experiment at Metal-Semiconductor Heterojunctions.

We propose an atomically resolved approach to capture the spatial variations of the Schottky barrier height (SBH) at metal-semiconductor heterojunctions. This proposed scheme, based on atom-specific partial density of states (PDOS) calculations, further enables calculation of the effective SBH that aligns with conductance measurements. We apply this approach to study the variations of SBH at MoS2@Au heterojunctions, in which MoS2 contains conducting and semiconducting grain boundaries (GBs). Our results reveal that there are significant variations in SBH at atoms in the defected heterojunctions. Of particular interest is the fact that the SBH in some areas with extended defects approaches zero, indicating Ohmic contact. One important implication of this finding is that the effective SBH should be intrinsically dependent on the defect density and character. Remarkably, the obtained effective SBH values demonstrate good agreement with existing experimental measurements. Thus, the present study addresses two long-standing challenges associated with SBH in MoS2-metal heterojunctions: the wide variation in experimentally measured SBH values at MoS2@metal heterojunctions and the large discrepancy between density-functional-theory-predicted and experimentally measured SBH values. Our proposed approach points out a valuable pathway for understanding and manipulating SBHs at metal-semiconductor heterojunctions.

Understanding the Electronic Transport of Al-Si and Al-Ge Nanojunctions by Exploiting Temperature-Dependent Bias Spectroscopy.

Understanding the electronic transport of metal-semiconductor heterojunctions is of utmost importance for a wide range of emerging nanoelectronic devices like adaptive transistors, biosensors, and quantum devices. Here, we provide a comparison and in-depth discussion of the investigated Schottky heterojunction devices based on Si and Ge nanowires contacted with pure single-crystal Al. Key for the fabrication of these devices is the selective solid-state metal-semiconductor exchange of Si and Ge nanowires into Al, delivering void-free, single-crystal Al contacts with flat Schottky junctions, distinct from the bulk counterparts. Thereof, a systematic comparison of the temperature-dependent charge carrier injection and transport in Si and Ge by means of current-bias spectroscopy is visualized by 2D colormaps. Thus, it reveals important insights into the operation mechanisms and regimes that cannot be exploited by conventional single-sweep output and transfer characteristics. Importantly, it was found that the Al-Si system shows symmetric effective Schottky barrier (SB) heights for holes and electrons, whereas the Al-Ge system reveals a highly transparent contact for holes due to Fermi level pinning close to the valence band with charge carrier injection saturation due to a thinned effective SB. Moreover, thermionic field emission limits the overall electron conduction, indicating a distinct SB for electrons.

Dual-plasmonic CuS@Au heterojunctions synergistic enhanced photothermal and colorimetric dual signal for sensitive multiplexed LFIA

Multiplexed immunodetection, which achieves qualitative and quantitative outcomes for multiple targets in a single-run process, provides more sufficient results to guarantee food safety. Especially, lateral flow immunoassay (LFIA), with the ability to offer multiple test lines for analytes and one control line for verification, is a forceful candidate in multiplexed immunodetection. Nevertheless, given that single-signal mode is incredibly vulnerable to interference, further efforts should be engrossed on the combination of multiplexed immunodetection and multiple signals. Photothermal signal has sparked significant excitement in designing immunosensors. In this work, by optimizing and comparing the amount of gold, CuS@Au heterojunctions (CuS@Au HJ) were synthesized. The dual-plasmonic metal-semiconductor hybrid heterojunction exhibits a synergistic photothermal performance by increasing light absorption and encouraging interfacial electron transfer. Meanwhile, the colorimetric property is synergistic enhanced, which is conducive to reduce the consumption of antibodies and then improve assay sensitivity. Therefore, CuS@Au HJ are suitable to be constructed in a dual signal and multiplexed LFIA (DSM-LFIA). T-2 toxin and deoxynivalenol (DON) were used as model targets for the simulated multiplex immunoassay. In contrast to colloidal gold-based immunoassay, the built-in sensor has increased sensitivity by ≈ 4.42 times (colorimetric mode) and ≈17.79 times (photothermal mode) for DON detection and by ≈ 1.75 times (colorimetric mode) and ≈13.09 times (photothermal mode) for T-2 detection. As a proof-of-concept application, this work provides a reference to the design of DSM-LFIA for food safety detection.

Metal-semiconductor heterojunction accelerates the plasmonically powered photoregeneration of biological cofactors.

Photocatalysis with plasmonic nanoparticles (NPs) is emerging as an attractive strategy to make and break chemical bonds. However, the fast relaxation dynamics of the photoexcited charge carriers in plasmonic NPs often result in poor yields. The separation and extraction of photoexcited hot-charge carriers should be faster than the thermalization process to overcome the limitation of poor yield. This demands the integration of rationally chosen materials to construct hybrid plasmonic photocatalysts. In this work, the enhanced photocatalytic activity of gold nanoparticle-titanium dioxide metal-semiconductor heterostructure (Au-TiO2) is used for the efficient regeneration of nicotinamide (NADH) cofactors. The modification of plasmonic AuNPs with n-type TiO2 semiconductor enhanced the charge separation process, because of the Schottky barrier formed at the Au-TiO2 heterojunction. This led to a 12-fold increment in the photocatalytic activity of plasmonic AuNP in regenerating NADH cofactor. Detailed mechanistic studies revealed that Au-TiO2 hybrid photocatalyst followed a less-explored light-independent pathway, in comparison to the conventional light-dependent path followed by sole AuNP photocatalyst. NADH regeneration yield reached ~70% in the light-independent pathway, under optimized conditions. Thus, our study emphasizes the rational choice of components in hybrid nanostructures in dictating the photocatalytic activity and the underlying reaction mechanism in plasmon-powered chemical transformations.

Highly efficient electrolytic reduction of nitrate to produce ammonia using Cu@Ni2P-NF Schottky heterojunction

Electrolytic reduction of nitrate to produce ammonia (NO3-RR) is one of the very promising alternatives to the energy-consuming Haber-Bosch process. The catalytic activity of nitrate reduction can be effectively improved by tuning the electronic structure and interfacial electron transfer. In this work, Cu@Ni2P-NF metal- semiconductor heterostructure is designed to adjust the adsorption of H* while enhance the adsorption of NO3- through schottky contacts. It is shown that the synergistic effects realized by the built-in electric field could effectively improve the NO3-RR catalytic performance. In the electrolytic cell with 1 M KOH and 20 mM KNO3, the ammonia yield at –0.2 V (vs. RHE) reaches 0.374 mmol∙h−1∙cm−2, and the nitrate removal efficiency is up to 94.63% with a selectivity of 90.25%. The energy conversion is realized by applying a Zn-NO3- battery with power density of 5.09 mW∙cm−2. This work demonstrates that the design of metal-semiconductor schottky heterojunction is an effective way to obtain novel electrocatalysts for NO3-RR.

Electrical contact properties of 2D metal-semiconductor heterojunctions composed of different phases of NbS<sub>2</sub> and GeS<sub>2</sub>

Metal-semiconductor heterojunction (MSJ) is the basis for developing novel devices. Here, we consider different two-dimensional van der Waals MSJs consisting of different-phase metals H- and T-NbS<sub>2</sub> and semiconductor GeS<sub>2</sub>, and conduct an in-depth study of their structural stabilities, electronic and electrical contact properties, with an emphasis on exploring the dependence of the electrical contact properties of the MSJs on the different phases of metals. Calculation results of their binding energy, phonon spectra, AIMD simulations, and mechanical properties show that both heterojunctions are highly stable, which implies that it is possible to prepare them experimentally and feasible to use them for designing electronic devices. The intrinsic H-NbS<sub>2</sub>/GeS<sub>2</sub> and T-NbS<sub>2</sub>/GeS<sub>2</sub> heterojunctions form p-type Schottky contacts and quasi-n-type Ohmic contacts, respectively. It is also found that their Schottky barrier heights (SBHs) and electrical contact types can be effectively modulated by an applied electric field and biaxial strain. For example, for the H-NbS<sub>2</sub>/GeS<sub>2</sub> heterojunction, Ohmic contact can be achieved regardless of applying a positive/negative electric field or planar biaxial compression, while for the T-NbS<sub>2</sub>/GeS<sub>2</sub> heterojunction, Ohmic contact can be achieved only at a very low negative electric field. The planar biaxial stretching can achieve quasi-Ohmic contact. In other words, when the semiconductor GeS<sub>2</sub> monolayer is used as the channel material of the field effect transistor and contacts different metal NbS<sub>2</sub> monolayers to form the MSJ, the interfacial Schottky barriers are distinctly different, and each of them has its own advantages in different situations (intrinsic or physically regulated). Therefore, this study is of great significance for understanding the physical mechanism of the electrical contact behaviors for H(T)-NbS<sub>2</sub>/GeS<sub>2</sub> heterojunction, especially for providing the theoretical reference for selecting suitable metal electrodes for the development of high-performance electronic devices.

Light concentration and electron transfer in plasmonic-photonic Ag,Au modified Mo-BiVO4 inverse opal photoelectrocatalysts.

Plasmonic photocatalysis based on metal-semiconductor heterojunctions is considered a key strategy to evade the inherent limitations of poor light harvesting and charge separation of semiconductor photocatalysts. It can be profitably combined with three-dimensional photonic crystals (PCs) that offer an ideal scaffold for loading plasmonic nanoparticles and a unique architecture to intensify photon capture. In this work, Mo-doped BiVO4 inverse opals were applied as visible light-responsive photonic hosts of Ag and/or Au plasmonic nanoparticles in order to exploit the synergy of plasmonic and photonic amplification effects with interfacial charge transfer for the photoelectrocatalytic degradation of recalcitrant pharmaceutical contaminants under visible light. Photoelectrochemical evaluation indicated a major contribution from hot spot-assisted local field enhancement, most pronounced for Ag/Mo-BiVO4 PCs due to the spectral overlap of the localized surface plasmon resonance with the electronic absorption and blue-edge slow photon region of Mo-BiVO4 PCs, in contrast to weak plasmonic sensitization effects for the Au-modified PCs. The diverse band alignment at the metal-semiconductor interfaces resulted in the enhanced photoelectrocatalytic degradation of tetracycline broad spectrum antibiotic by Ag/Mo-BiVO4 and the refractory ibuprofen drug by (Ag,Au)/Mo-BiVO4, attributed to the enhanced charge separation by electron transfer toward Ag nanoparticles. Combination of visible light activated semiconductor PCs and plasmonic nanoparticles with suitable band alignment and photonic band gap may provide a versatile approach for the rational design of efficient plasmonic-photonic photoeletrocatalysts.

Cu-(Ga0.2Cr0.2Mn0.2Ni0.2Zn0.2)3O4 heterojunction derived from high entropy oxide precursor and its photocatalytic activity for CO2 reduction with water vapor

The construction of metal–semiconductor heterojunction has been proven to be an effective way to improve the separation efficiency of photogenerated electrons and holes, thus enhancing photocatalytic activity of semiconductor with narrow band gap. Here, we construct a Cu-(Ga0.2Cr0.2Mn0.2Ni0.2Zn0.2)3O4 high entropy oxide (HEO) heterojunction by using Cu-containing HEO precursor, and the effect of Cu addition method on the photocatalytic performance was investigated. The results show that the addition method of Cu has a great influence on the size of Cu nanoparticles (NPs), heterojunction amount and photocatalytic activity of Cu-HEO catalysts. Cu-HEO samples prepared by direct introduction method possess much smaller Cu particles due to the uniform distribution and stronger diffusion hysteresis effect of Cu in HEO, and exhibit superior photocatalytic activity and CH4 selectivity for CO2 reduction. With the increase of Cu loading, the photocatalytic activity of Cu-HEO samples firstly increases and then decreases. Among these photocatalysts, Cu0.5-HEO obtained by the direct introduction method displays the highest CO2 reduction activity, and the yields of CO and CH4 reach to 5.66 and 33.84 µmol/h/gcat, respectively. This work innovatively uses HEO precursors with unique properties to construct metal-HEO heterojunctions with smaller metal particles and more active sites.

Surface enhanced Raman scattering based on ZnO/Cu@Ag heterojunction for detecting γ-aminobutyric acid molecules

Detecting γ-aminobutyric acid (GABA) in the mammalian body is vital for diagnosing neurological disorders. Here, we introduced a novel SERS sensor based on metal-semiconductor heterojunction ZnO/Cu@Ag substrate for highly sensitive detecting GABA molecules. The ZnO/Cu@Ag substrate fully leverages the electromagnetic field enhancement effect and the chemical enhancement effect of metal-semiconductors heterojunction, exhibiting an excellent enhancement factor of 2.23 × 106 and a low limit-of-detection (LOD) of 3.84 nM for GABA molecules in water. Moreover, the ZnO/Cu@Ag substrate could recognize 10 nM GABA molecules in human serum. This GABA SERS sensor provides a reliable tool and technological support for research and applications in neuroscience and medicine.

Mott Schottky heterojunction Co/CoSe2 electrocatalyst: Achieved rapid conversion of polysulfides and Li2S deposition dissolution via built-in electric field interface effect

Rechargeable lithium-sulfur (Li-S) batteries are considered ideal candidates for the next generation of energy storage devices. However, the insulation properties of S and Li2S, as well as the notorious “shuttle effect” of lithium polysulfide, result in severe loss of active sulfur, poor redox kinetics, and rapid capacity decline. To overcome these limitations, this paper designs a self-supporting electrocatalytic electrode based on Co/CoSe2 Mott Schottky heterojunction coated carbon nanofibers (Co/CoSe2@NCNFs), and engineers it for capturing and converting polysulfides. Density functional theory calculations show that the introduction of Co/CoSe2 heterojunction effectively reduces the redox barrier of Li2S. The electron current and electron interaction at the Co/CoSe2 interface induce the redistribution of charge density in the depletion region, thus increasing the chemical reactivity of the metal semiconductor heterojunction interface, which is conducive to the ion/electron transport, polysulfide conversion and Li2S oxidation process. Therefore, the Li-S battery assembled by Co/CoSe2@NCNFs/S electrode exhibited a high initial specific capacity of 1043.3 mAh/g at 1C, and maintained a capacity of 709.9 mAh/g after 1000 cycles, with a capacity decay rate of only 0.032% per cycle. The proposed Mott Schottky heterojunction as an electrocatalyst deepens the promotion of polysulfides and Li2S in long-life Li-S batteries.

High-performance photodetection of UV-visible-NIR by Ag–Mn2O3 heterojunctions

Ag–Mn2O3 was evaluated as a new photosensing material to detect UV–visible–NIR radiation, and the results were compared to those obtained from single-phase Mn2O3. The materials were prepared by the coprecipitation method, from metal nitrates, and later calcined at 500 °C, in air. The morphology of Ag–Mn2O3 consists of cuboid-shaped microparticles, with silver nanowires randomly attached to its surface. The photoresponse profiles show uniform and reproducible variations in electrical current caused by light exposure. However, for Ag–Mn2O3 they were up to 46 times higher than those measured in Mn2O3. This improvement can be attributed to the decrease in the bandgap energy of Ag–Mn2O3, associated with the formation of metal-semiconductor heterojunctions and the fact that silver has a lower work function and a greater number of free electrons.

On-Chip Monolithically Integrated UltravioletLow-Threshold Plasmonic Metal-Semiconductor Heterojunction Nanolasers.

The metal-semiconductor heterojunction is imperative for the realization of electrically driven nanolasers for chip-level platforms. Progress in developing such nanolasers has hitherto rarely been realized, however, because of their complexity in heterojunction fabrication and the need to use noble metals that are incompatible with microelectronic manufacturing. Most plasmonic nanolasers lase either above a high threshold (101 -103 MW cm-2 ) or at a cryogenic temperature, and lasing is possible only after they are removed from the substrate to avoid the large ohmic loss and the low modal reflectivity, making monolithic fabrication impossible. Here, for the first time, record-low-threshold, room-temperature ultraviolet (UV) lasing of plasmon-coupled core-shell nanowires that are directly grown on silicon is demonstrated. The naturally formed core-shell metal-semiconductor heterostructure of the nanowires leads to a 100-fold improvement in growth density over previous results. This unprecedentedly high nanowire density creates intense plasmonic resonance, which is outcoupled to the resonant Fabry-Pérot microcavity. By boosting the emission strength by a factor of 100, the hybrid photonic-plasmonic system successfully facilitates a record-low laser threshold of 12kW cm-2 with a spontaneous emission coupling factor as high as ≈0.32 in the 340-360nm range. Such architecture is simple and cost-competitive for future UV sources in silicon integration.

Facile hydrothermal process achieving 1T/2H phase tailoring of MoS2 nanosheets for efficient microwave absorption

Electromagnetic pollution is becoming a serious concern. Electromagnetic wave absorbers with light weight, high efficiency and broadband are desirable for electromagnetic protection. Recently, 1T/2H hybrid molybdenum disulfide nanosheets attracted great attention due to the enhanced microwave attenuation driven from fabrication of 1T/2H metal-semiconductor heterojunctions. However, the easy and controllable 1T/2H phase tailoring remains a challenge because of the difficulty in accurately controlling the two-phase ratio and the complicated process. In this study, a facile hydrothermal process is proposed to acquire the 1T/2H MoS2 nanosheets with different 1T contents. The high-resolution TEM images reveal the MoS2 nanosheets are composed of 1T/2H nano-heterojunctions with the 1T and 2H nanodomains arranged alternatively. Without compositing with conductive matters, the MoS2 nanosheets exhibit excellent microwave absorption performance with a maximum reflection loss of −27 dB and an effective absorption bandwidth of ∼4.16 GHz (at 2.3 mm thickness). It benefits from the increased conductivity of the MoS2 nanosheets, abundant 1T/2H phase boundaries and lattice defects, and improved impedance matching induced by the embedding of the 1T phase in the 2H matrix. This work demonstrates the feasibility of the proposed facile hydrothermal process in achieving 1T/2H phase regulation, providing an effective method to tailor the properties of MoS2 nanosheets for various applications.

Near-infrared upconversion photosensitizer enabling photodynamic production and in situ dynamic monitoring of hydroxyl radical in live mice

Upconversion photodynamic therapy (PDT), utilizing upconversion nanoparticles (UCNPs) to excite surface-anchored molecular photosensitizers, enables to treat deep-seated tumors using tissue-penetrating near-infrared (NIR) light. Yet, this technique was substantially limited by the O2 deficiency in solid tumors and the inability to visualize reactive oxygen species (ROS) for precise treatment. To address this challenge, we devised a neotype of multifunctional upconversion photosensitizer which not only efficiently generates hydroxyl radical (•OH) to induce cell apoptosis in hypoxic tumor, but also dynamically shows accumulated amount of radical •OH in situ in tumor. This photosensitizer consists of NaYbF4:Tm@NaYF4 upconversion nanoparticles (UCNPs) as the core, and mesoporous silica as the shell which was loaded with heterojunction Ag0-Ag2S quantum dots and IR 820 molecular probe on the surface. Under 980 nm NIR excitation, thulium ultraviolet and blue upconversion luminescence (UCL) excite spectrally overlapped Ag0-Ag2S, with metal-semiconductor heterojunction for enhanced electron-hole separation, to produce highly toxic •OH to induce cell death. While produced •OH can specifically degrade surface IR 820 dyes to remove luminescence resonant energy transfer from UCNPs to IR 820 dyes, this can turn on quenched UCL entailing dynamical observation of in situ generated •OH. Intravenous injection of the photosensitizers resulted in 8-fold tumor volume reduction as compared to control group, while dynamically showing accumulated production of •OH in hypoxic 4T1 tumor-bearing mice (Balb/c). These findings untap the use of upconversion photosensitizer for precise and visualized treatment of deep-seated hypoxic tumors in live mammals.

Atomic Layer Deposition of Ultra-Thin Crystalline Electron Channels for Heterointerface Polarization at Two-Dimensional Metal-Semiconductor Heterojunctions

Atomic layer deposition (ALD) has emerged as a promising technology for the development of the next generation of low-power semiconductor electronics. The wafer-scaled growth of two-dimensional (2D) crystalline nanostructures is a fundamental step toward the development of advanced nanofabrication technologies. Ga2O3 is an ultra-wide bandgap metal oxide semiconductor for application in electronic devices. The polymorphous Ga2O3 with its unique electronic characteristics and doping capabilities is a functional option for heterointerface engineering at metal-semiconductor 2D heterojunctions for application in nanofabrication technology. Plasma-enhanced atomic layer deposition (PE-ALD) enabled the deposition of ultra-thin nanostructures at low-growth temperatures. The present study used the PE-ALD process for the deposition of atomically thin crystalline ß-Ga2O3 films for heterointerface engineering at 2D metal-semiconductor heterojunctions. Via the control of plasma gas composition and ALD temperature, the wafer-scaled deposition of ~5.0 nm thick crystalline ß-Ga2O3 at Au/Ga2O3-TiO2 heterointerfaces was achieved. Material characterization techniques showed the effects of plasma composition and ALD temperature on the properties and structure of Ga2O3 films. The following study on the electronic characteristics of Au/Ga2O3-TiO2 2D heterojunctions confirmed the tunability of this metal/semiconductor polarized junction, which works as functional electron channel layer developed based on tunable p-n junctions at 2D metal/semiconductor interfaces.

Large-Area Metal-Semiconductor Heterojunctions Realized via MXene-Induced Two-Dimensional Surface Polarization.

Direct MXene deposition on large-area 2D semiconductor surfaces can provide design versatility for the fabrication of MXene-based electronic devices (MXetronics). However, it is challenging to deposit highly uniform wafer-scale hydrophilic MXene films (e.g., Ti3C2Tx) on hydrophobic 2D semiconductor channel materials (e.g., MoS2). Here, we demonstrate a modified drop-casting (MDC) process for the deposition of MXene on MoS2 without any pretreatment, which typically degrades the quality of either MXene or MoS2. Different from the traditional drop-casting method, which usually forms rough and thick films at the micrometer scale, our MDC method can form an ultrathin Ti3C2Tx film (ca. 10 nm) based on a MXene-introduced MoS2 surface polarization phenomenon. In addition, our MDC process does not require any pretreatment, unlike MXene spray-coating that usually requires a hydrophilic pretreatment of the substrate surface before deposition. This process offers a significant advantage for Ti3C2Tx film deposition on UV-ozone- or O2-plasma-sensitive surfaces. Using the MDC process, we fabricated wafer-scale n-type Ti3C2Tx-MoS2 van der Waals heterojunction transistors, achieving an average effective electron mobility of ∼40 cm2·V-1·s-1, on/off current ratios exceeding 104, and subthreshold swings of under 200 mV·dec-1. The proposed MDC process can considerably enhance the applications of MXenes, especially the design of MXene/semiconductor nanoelectronics.

Ultrathin All-2D Lateral Diodes Using Top and Bottom Contacted Laterally Spaced Graphene Electrodes to WS2 Semiconductor Monolayers

The ultrathin nature of two-dimensional (2D) materials opens up opportunities for creating devices that are substantially thinner than using traditional bulk materials. In this article, monolayer 2D materials grown by the chemical vapor deposition method are used to fabricate ultrathin all-2D lateral diodes. We show that placing graphene electrodes below and above the WS2 monolayer, instead of the same side, results in a lateral device with two different Schottky barrier heights. Due to the natural dielectric environment, the bottom graphene layer is wedged between the WS2 and the SiO2 substrate, which has a different doping level than the top graphene layer that is in contact with WS2 and air. The lateral separation of these two graphene electrodes results in a lateral metal-semiconductor-metal junction with two asymmetric barriers but yet retains its ultrathin form of two-layer thickness. The rectification and diode behavior can be exploited in transistors, photodiodes, and light-emitting devices. We show that the device exhibits a rectification ratio up to 90 under a laser power of 1.37 μW at a bias voltage of ±3 V. We demonstrate that both the back-gate voltage and laser illumination can tune the rectification behavior of the device. Furthermore, the device can generate strong red electroluminescence in the WS2 area across the two graphene electrodes under an average flowing current of 2.16 × 10-5 A. This work contributes to the current understanding of the 2D metal-semiconductor heterojunction and offers an idea to obtain all-2D Schottky diodes by retaining the ultrathin device concept.