Space-confined Chemical Vapor Deposition Research Articles

Homogeneous Mo1−xRexS2 monolayer and lateral 1T’@2H heterostructures for significantly enhanced and highly reproducible SERS responses and mechanistic insights

As a promising surface-enhanced Raman scattering (SERS) substrate, two-dimensional (2D) nanomaterials ensure the uniformity and reproducibility of the SERS response. However, their extremely flat surfaces are not beneficial for high-capacity binding of analytes and significant enhancement of Raman signals. In this contribution, monolayer Mo0.27Re0.73S2 alloys with distorted lattice and modulated optoelectronic structure are facilely fabricated by space-confined chemical vapor deposition (CVD), exhibiting a significantly enhanced SERS performance with an enhancement factor of 2.0 × 1011. As demonstrated, the continuous increment of Re ratio results in a phase transition from 2H to 1T’ for homogeneous Mo1−xRexS2 monolayers. As a result, the 1T’-phase Mo0.27Re0.73S2 alloys achieve a sensitive detection of down to 10−15 M for Rhodamine 6G while the value on MoS2 or ReS2 is only 10−9 M. Additionally, the alloy-based SERS substrate demonstrates excellent SERS detection reproducibility, long-term stability, and universality. Referring to the space-confined CVD, a dynamics-controlled process is further employed for the in-situ fabrication of 1T’@2H Mo1−xRexS2 heterostructures, providing a new approach and insight for facilely building varied phases-based lateral heterostructures by the control of composition ratio. Overall, this work demonstrates the enormous potential of alloy engineering in improving the SERS performance of TMDs and the convenient fabrication of out-of-phase TMD alloys.

Atomic-Thin WS2 Kirigami for Bidirectional Polarization Detection.

The assembly and patterning engineering in two-dimensional (2D) materials hold importance for chip-level designs incorporating multifunctional detectors. At present, the patterning and stacking methods of 2D materials inevitably introduce impurity instability and functional limitations. Here, the space-confined chemical vapor deposition method is employed to achieve state-of-the-art kirigami structures of self-assembled WS2, featuring various layer combinations and stacking configurations. With this technique as a foundation, the WS2 nano-kirigami is integrated with metasurface design, achieving a photodetector with bidirectional polarization-sensitive detection capability in the infrared spectrum. Nano-kirigami can eliminate some of the uncontrollable factors in the processing of 2D material devices, providing a freely designed platform for chip-level multifunctional detection across multiple modules.

Space-Confined Growth of Ultrathin P-Type GeTe Nanosheets for Broadband Photodetectors.

As p-type phase-change degenerate semiconductors, crystalline and amorphous germanium telluride (GeTe) exhibit metallic and semiconducting properties, respectively. However, the massive structural defects and strong interface scattering in amorphous GeTe films significantly reduce their performance. In this work, two-dimensional (2D) p-type GeTe nanosheets are synthesized via a specially designed space-confined chemical vapor deposition (CVD) method, with the thickness of the GeTe nanosheets reduced to 1.9nm. The space-confined CVD method improves the crystallinity of ultrathin GeTe by lowering the partial pressure of the reactant gas, resulting in GeTe nanosheets with excellent p-type semiconductor properties, such as a satisfactory on/off ratio of 105. Temperature-dependent electrical measurements demonstrate that variable-range hopping and optical-phonon-assisted hopping mechanisms dominate transport behavior at low and high temperatures, respectively. GeTe devices exhibit significantly high responsivity (6589 and 2.2 A W-1 at 633 and 980nm, respectively) and detectivity (1.67 × 1011 and 1.3 × 108 Jones at 633 and 980nm, respectively), making them feasible for broadband photodetectors in the visible to near-infrared range. Furthermore, the fabricated GeTe/WS2 diode exhibits a rectification ratio of 103 at zero gate voltage. These satisfactory p-type semiconductor properties demonstrate that ultrathin GeTe exhibits enormous potential for applications in optoelectronic interconnection circuits.

In Situ Growth of High-Quality Single-Crystal Twisted Bilayer Graphene on Liquid Copper.

Twisted bilayer graphene (TBG) generates significant attention in the fundamental research of 2D materials due to its distinct twist-angle-dependent properties. Exploring the efficient production of TBG with a wide range of twist angles stands as one of the major frontiers in moiré materials. Here, the local space-confined chemical vapor deposition growth technique for high-quality single-crystal TBG with twist angles ranging from 0° to 30° on liquid copper substrates is reported. The clean surface, pristine interface, high crystallinity, and thermal stability of TBG are verified by using comprehensive characterization techniques including optical microscopy, electron microscopy, and secondary-ion mass spectrometry. The proportion of TBG in bilayer graphene reaches as high as 89%. In addition, the stacking structure and growth mechanism of TBG are investigated, revealing that the second graphene layer develops beneath the first one. A series of comparative experiments illustrates that the liquid copper surface, with its excellent fluidity, promotes the growth of TBG. Electrical measurements show the twist-angle-dependent electronic properties of as-grown TBG, achieving a room-temperature carrier mobility of 26640 cm2 V-1 s-1 . This work provides an approach for the in situ preparation of 2D twisted materials and facilitates the application of TBG in the fields of electronics.

Magnetic proximity effect in the two-dimensional ε-Fe2O3/NbSe2 heterojunction

Two-dimensional (2D) magnet/superconductor heterostructures can promote the design of artificial materials for exploring 2D physics and device applications by exotic proximity effects. However, plagued by the low Curie temperature and instability in air, it is hard to realize practical applications for the reported layered magnetic materials at present. In this paper, we developed a space-confined chemical vapor deposition method to synthesize ultrathin air-stable ε-Fe2O3 nanosheets with Curie temperature above 350 K. The ε-Fe2O3/NbSe2 heterojunction was constructed to study the magnetic proximity effect on the superconductivity of the NbSe2 multilayer. The electrical transport results show that the subtle proximity effect can modulate the interfacial spin–orbit interaction while undegrading the superconducting critical parameters. Our work paves the way to construct 2D heterojunctions with ultrathin nonlayered materials and layered van der Waals (vdW) materials for exploring new physical phenomena.

Hydrogen-assisted growth of one-dimensional tellurium nanoribbons with unprecedented high mobility

High-mobility van der Waals ambipolar semiconductors are promising in logic and reconfigurable circuits, integrated optoelectronic circuits, due to the excellent gate-controlled capability and effectively tunability of major charge carriers by electrostatic field. Controllable growth of high-quality ambipolar semiconductors with high mobility and stability is highly glamorous and indispensable for further research. Here, we demonstrate a straightforward space-confined chemical vapor deposition (CVD) method to synthesize high-quality quasi-one-dimensional (1D) tellurium (Te) nanoribbons (NRs). By introducing H2 into the gas flow, endothermic compound H2Te was generated from the reaction of liquid Te with H2, and consequently decomposed into elemental Te at low temperature. Further, the Te NRs have been utilized for in-situ fabrication of field-effect transistors (FETs) without transferring process. Ambipolar features are achieved using nickel (Ni) as an ohmic contact. More importantly, the mobilities of the Te NR transistor for hole/electron are as high as 1755/28.6 cm2V−1s−1 and 4024/278 cm2V−1s−1 at room temperature and under a temperature below 20 K, respectively. Our findings confirm the novel strategy for synthesizing 1D elemental semiconductors and their applications with ambipolar behaviors.

Room-Temperature Magnetoelectric Coupling in Atomically Thin ε-Fe2 O3.

2D multiferroics with magnetoelectric coupling combine the magnetic order and electric polarization in a single phase, providing a cornerstone for constructing high-density information storages and low-energy-consumption spintronic devices. The strong interactions between various order parameters are crucial for realizing such multifunctional applications, nevertheless, this criterion is rarely met in classical 2D materials at room-temperature. Here an ingenious space-confined chemical vapor deposition strategy is designed to synthesize atomically thin non-layered ε-Fe2 O3 single crystals and disclose the room-temperature long-range ferrimagnetic order. Interestingly, the strong ferroelectricity and its switching behavior are unambiguously discovered in atomically thin ε-Fe2 O3 , accompanied with an anomalous thickness-dependent coercive voltage. More significantly, the robust room-temperature magnetoelectric coupling is uncovered by controlling the magnetism with electric field and verifies the multiferroic feature of atomically thin ε-Fe2 O3 . This work not only represents a substantial leap in terms of the controllable synthesis of 2D multiferroics with robust magnetoelectric coupling, but also provides a crucial step toward the practical applications in low-energy-consumption electric-writing/magnetic-reading devices.

Two-dimensional bilayer MoS2 flakes with variable size of the upper layer grown by chemical vapor deposition

Bilayer transition-metal dichalcogenides, particularly twisted bilayer, provide a promising platform for the exploration of novel fundamental properties and applications in valleytronics and twistronics. Here, we report the growth of two-dimensional (2D) bilayer molybdenum disulfide (MoS2) flakes with varied upper-layer size by a conventional space-confined chemical vapor deposition. The area ratio of upper layer/bottom layer of these bilayer flakes at the difference locations along the gas-flow direction is estimated to be 5% (S1), 35% (S2), 73% (S3), and 100% (S4), respectively. The detailed layer number and optical properties of these samples have been evaluated by AFM, Raman, and PL characterizations. Furthermore, the evolution of area ratio of upper layer/bottom layer is unveiled by the energy and precursors concentration variation along the gas flow direction during the narrow space. Our results provide a feasible approach for the fabrication of 2D bilayer TMDCs crystals that can accelerate the performance study and promising applications.

Facile fabrication of aluminum-oxide deposited CuO/CaO composites with enhanced stability and CO2 capture capacity for combined Ca/Cu looping process

The combined Ca–Cu looping process is a promising CO 2 capture technology, in which the exothermic reduction of CuO with methane is used in situ to provide the heat required to calcine CaCO 3 . However, the CO 2 capture capacity of CaO/CuO composites decreases rapidly during multiple cycles. Herein, we proposed an aluminum-oxide deposited CaO/CuO composites synthesized by the space-confined chemical vapor deposition method and systematically investigated in accordance carbonation, calcination-reduction, and oxidation cycles. The deposited CaO/CuO composites feature highly porous structures and fine grain sizes, demonstrating significantly enhanced CO 2 capture performance with only 5 mol.% of Al stabilizer. The CO 2 absorption amount of Cu 1 CaAl 0.05 is 0.2183 g CO2 /g material in the first cycle. After 20 cycles, it still maintains 86.6% of the original absorption capacity, exceeding that of the undeposited CaO/CuO composites by 115%. Moreover, during the 20 cycles, the oxidation conversion of Cu 1 CaAl 0.05 always keeps above 95%, much higher than undeposited CaO/CuO composites. Various evidences indicate that the uniform deposition of aluminum-oxide on the CaO/CuO sorbent plays a vital role in improving the CO 2 absorption performance and cycling stability. • Al-stabilized CuO/CaO composites was synthesized by space-confined chemical vapor deposition. • Al-deposition optimized the physical structure of the adsorbent. • Only 5 mol.% Al stabilizer was used to obtain excellent performance. • The obtained adsorbent exhibited excellent stability in 20 cycles.

Two-Dimensional Room-Temperature Magnetic Nonstoichiometric Fe7Se8 Nanocrystals: Controllable Synthesis and Magnetic Behavior.

Two-dimensional (2D) magnetic materials have attracted significant attention for promising applications in energy-saving logic and robust memory devices. However, most 2D magnets discovered so far typically feature drawbacks for practical applications due to low critical temperatures. Herein, we synthesize ultrathin room-temperature (RT) magnetic Fe7Se8 nanoflakes via the space-confined chemical vapor deposition method. It is found that the appropriate supply and control of Se concentration in the reaction chamber is crucial for synthesizing high-quality nonstoichiometric Fe7Se8 nanoflakes. Cryogenic electrical and magnetic characterizations reveal the emergence of spin reorientation at ∼130 K and the survival of long-range magnetic ordering up to room temperature. The RT magnetic domain structures with different thicknesses are also uncovered by magnetic force microscopy. Moreover, theoretical calculations confirm the spin configuration and metallic band structure. The outstanding characteristics exhibited by Fe7Se8 nanoflakes, including RT magnetism, spin reorientation property, and good electrical conductivity, make them a potential candidate for RT spintronics.

Growth and Optoelectronic Properties of Large-Scale Bilayer WS2 Ribbons with Unusual Shapes via Chemical Vapor Deposition

Atomically thin transition metal dichalcogenides (TMDCs) have attracted worldwide interest for their prospective potential in electronic and optoelectronic fields. However, TMDCs tend to grow into two-dimensional (2D) flakes rather than one-dimensional (1D) micro/nanoribbons due to their intrinsic layered structure. Here, large-scale bilayer WS2 ribbons with unusual shapes have been achieved through the space-confined chemical vapor deposition method via utilizing WO3 and S powders as the precursors. The microstructures and optical and electronic properties of as-grown bilayer WS2 ribbons have been systematically researched. Moreover, the possible growth mechanism of such special structures is revealed by the growth-time-dependent morphological evolution of the WS2 crystals. Our results emphasize the prospects for the successful fabrication of novel 1D TMDC structures for high-performance optoelectronic devices at nanoscale, including transistors, photocatalysts, photovoltaic cells, and spintronic devices.

The synthesis and identification of complete stacking bilayer MoS2 flakes with unconventional shapes via chemical vapor deposition

Two-dimensional complete stacking bilayer molybdenum disulfide (MoS 2 ) crystals with several unconventional morphologies, including four-point star, hexagon, rhomboid, trapezoid, and pentagon, have been attained by employing the space-confined chemical vapor deposition approach. The morphology, structure, and fundamental features of as-produced domains are systematically characterized via utilizing optical microscopy, atomic force microscopy , and Raman/PL system, elucidating bilayer feature of various MoS 2 flakes. The variation of such various bilayer MoS 2 crystals is discussed based on the difference of growth rate between different sides. • Complete stacking bilayer MoS 2 flakes with various shapes have been attained via space-confined chemical vapor deposition. • Evolution of varied shapes, layer-number, and optical properties of bilayer MoS 2 flakes has been evaluated. • Possible growth mechanism of these bilayer MoS 2 flakes with various shapes has been discussed.

Highly crystalline Mo1-xRexS2 monolayers by NaCl-assisted and space-confined chemical vapor deposition

Alloy engineering is of great importance in expanding the two-dimensional (2D) material family and modulating the bandgap of atomically thin 2D transition metal dichalcogenides (TMDs). Here, we report the synthesis of large area and high quality 2H Mo1-xRexS2 (x<=0.02) monolayer crystals by NaCl-assisted and space-confined chemical vapor deposition strategy. The combined effects of both common salt and confined space help promote the growth of Mo1-xRexS2 alloy monolayer crystals. The as-grown Mo1-xRexS2 alloy monolayers are uniformly distributed on the whole growth substrates with a domain size of up to 90 μm. Optical images, X-ray spectroscopy, Raman and photoluminescence spectroscopy measurements and corresponding mapping figures reflect the high quality and uniformity of the monolayers. The Mo1-xRexS2 alloy based phototransistor device exhibit good photoresponse to visible light with a response time of less than 0.2 s. Such alloy engineering of atomically thin 2D TMDs is useful for their future applications in optoelectronics.

Space-confined CVD growth of 2D-MoS2 crystals with tunable dimensionality via adjusting growth conditions

2D MoS2 crystals with tunable dimensionality can be realized by the reaction of S and Mo foil under adjusted growth conditions via a space-confined chemical vapor deposition method.

Development of dense Ca-based, Al-stabilized composites with high volumetric energy density for thermochemical energy storage of concentrated solar power

Thermochemical energy storage based on the Calcium-Looping process characterized high energy density, low cost, and scalability, which is an advantageous candidate for the heat storage systems of Concentrated Solar Power plants. The main disadvantage of Calcium-Looping technology is the rapid decrease in the reactivity of CaO during cycling due to sintering. Many methods have been developed to synthesize high-cycling-behavior CaO-based materials. However, the state-of-the-art synthesized CaO-based materials remain far from the requirements due to the lack of consideration from an application perspective, which not only demands high-cycling-behavior but also needs to consider volumetric energy density. In this work, dense CaO/Al2O3 composites were fabricated using the space-confined chemical vapor deposition method and considering both stability and volumetric energy density. Among different Ca-precursors, calcium formate does not expand during calcination, thus possesses a relatively dense structure, and taken together with the deposition of Al2O3 on the surface of CaO grains obtained composites possessing high density and excellent stability. The optimized composite with 10 mol.% Al achieves a high volumetric energy density of 2.07 GJ/m3 after 20 cycles, which is 1.58, 4.31, and 6.68 times of the capacity of limestone-derived CaO, sol–gel based CaO/Al2O3 composites, and wet-mixing based CaO/Al2O3 composites, respectively. This study presents a design principle for CaO-based heat storage materials, which are becoming denser and more stable.

Controllable preparation of ultrathin 2D BiOBr crystals for high-performance ultraviolet photodetector

Ternary layered compound materials (bismuth oxyhalides and metal phosphorus trichalcogenides) stand out in electronic and optoelectronic fields due to their interesting physical properties. However, few studies focus on the preparation of high-quality two-dimensional (2D) BiOBr crystals with a typical layered structure, let alone their optoelectronic applications. Here, for the first time, high-quality 2D BiOBr crystals with ultrathin thicknesses (less than 10 nm) and large domain sizes (∼100 µm) were efficiently prepared via a modified space-confined chemical vapor deposition (SCCVD) method. It is demonstrated that a moderate amount of H2O molecules in the SCCVD system greatly promote the formation of high-quality 2D BiOBr crystals because of the strong polarity of H2O molecules. In addition, a linear relationship between the thickness of BiOBr nanosheets and Raman shift of $${\rm{A}}_{1{\rm{g}}}^{\left( 1 \right)}$$ mode was found. Corresponding theoretical calculations were carried out to verify the experimental data. Furthermore, the BiOBr-based photodetector was fabricated, exhibiting excellent performances with a responsivity of 12.4 A W−1 and a detectivity of 1.6×1013 Jones at 365 nm. This study paves the way for controllable preparation of high quality 2D BiOBr crystals and implies intriguing opportunities of them in op toelectronic applications.

Ultrabroadband Photodetectors up to 10.6 µm Based on 2D Fe3O4 Nanosheets

The ultrabroadband spectrum detection from ultraviolet (UV) to long-wavelength infrared (LWIR) is promising for diversified optoelectronic applications of imaging, sensing, and communication. However, the current LWIR-detecting devices suffer from low photoresponsivity, high cost, and cryogenic environment. Herein, a high-performance ultrabroadband photodetector is demonstrated with detecting range from UV to LWIR based on air-stable nonlayered ultrathin Fe3 O4 nanosheets synthesized via a space-confined chemical vapor deposition (CVD) method. Ultrahigh photoresponsivity (R) of 561.2 A W-1 , external quantum efficiency (EQE) of 6.6 × 103 %, and detectivity (D*) of 7.42 × 108 Jones are achieved at the wavelength of 10.6 µm. The multimechanism synergistic effect of photoconductive effect and bolometric effect demonstrates the high sensitivity for light with any light intensities. The outstanding device performance and complementary mixing photoresponse mechanisms open up new potential applications of nonlayered 2D materials for future infrared optoelectronic devices.

High-performance UV detectors based on 2D CVD bismuth oxybromide single-crystal nanosheets

Two-dimensional (2D) ternary wide-bandgap semiconducting materials have great potential in power device, flexible electronic device, short-wavelength light emitting diodes (LEDs) and photodetectors due to the controllable bandgap, strong light-material interaction, and controlled freedom degree of stoichiometry variation. However, it is still a great challenge to precisely control the growth of high-quality 2D ternary wide-bandgap semiconducting materials due to the variety of components, which hinders their development for practical applications. In this work, high-quality 2D ternary bismuth oxybromide single-crystal nanosheets with a high yield were prepared by space-confined chemical vapor deposition (CVD) method. The devices based on 2D ultrathin BiOBr single-crystal nanoflakes show a high UV detecting performance including low dark current (Idark) of 1.46 pA and high responsivity (R), external quantum efficiency (EQE) and detectivity (D*) of 14.96 A W−1, 5460%, and 5.74 × 1010 Jones, respectively, as well as fast response process (τrise =80 ms, τdecay =40 ms). The excellent UV performance can be ascribed to the photogating effect by trapped states, which endow it with great potential for high-performance UV detectors.

High-performance CaO-based composites synthesized using a space-confined chemical vapor deposition strategy for thermochemical energy storage

The Calcium-Looping (CaL) process, based on the reversible carbonation/calcination of CaO, is a promising technology for thermochemical energy storage (TCES) in Concentrated Solar Power (CSP) plants. However, the major drawback of this technology is the rapid deactivation of CaO due to sintering. In this work, we have synthesized CaO-based, inert oxide-stabilized composites through a space-confined chemical vapor deposition process. Different synthesis parameters such as the inert material (Al2O3, SiO2 or TiO2) used as the stabilizer and the CaO concentration in the sorbent were investigated. Among the three different promoters used to increase the resistance of CaO toward sintering, Al2O3 resulted in the most stable composites. The composite with only 5 mol.% Al showed an excellent CO2 uptake capacity, 0.59 g CO2/g composite, over 50 cycles without any deactivation. Based on the calculation, this composite maintained an energy density of 1.50 GJ/t after 50 cycles, corresponding to 87% of the theoretical maximum. This high stability is attributed to the unique synthetic strategy in which the thermally stable oxide nanoparticles deposited on the surface of CaO crystalline grain and prevented its aggregation and overgrowth.

Nonlayered Two-Dimensional Defective Semiconductor γ-Ga2S3 toward Broadband Photodetection.

Two-dimensional (2D) materials exhibit high sensitivity to structural defects due to the nature of interface-type materials, and the corresponding structural defects can effectively modulate their inherent properties in turn, giving them a wide application range in high-performance and functional devices. 2D γ-Ga2S3 is a defective semiconductor with outstanding optoelectronic properties. However, its controllable preparation has not been implemented yet, which hinders exploring its potential applications. In this work, we introduce nonlayered γ-Ga2S3 into the 2D materials family, which was successfully synthesized via the space-confined chemical vapor deposition method. Its intriguing defective structure are revealed by high-resolution transmission electron microscopy and temperature-dependent cathodoluminescence spectra, which endow the γ-Ga2S3-based device with a broad photoresponse from the ultraviolet to near-infrared region and excellent photoelectric conversion capability. Simultaneously, the device also exhibits excellent ultraviolet detection ability ( Rλ = 61.3 A W-1, Ion /Ioff = 851, EQE = 2.17× 104 %, D* = 1.52× 1010 Jones @350 nm), and relatively fast response (15 ms). This work provides a feasible way to fabricate ultrathin nonlayered materials and explore the potential applications of a 2D defective semiconductor in high-performance broadband photodetection, which also suggests a promising future of defect creation in optimizing photoelectric properties.