Two-step Chemical Vapor Deposition Method Research Articles

Two-Step Chemical Vapor Deposition for Fabrication of FAPbI3 Single-Crystal Microsheets with High Exciton Binding Energy.

Hybrid perovskites exhibit highly efficient optoelectronic properties and find widespread applications in areas such as solar cells, light-emitting diodes, photodetectors, and lasers. Here, we report the innovative synthesis of formamidinium lead iodide (FAPbI3) single-crystal microsheets via a two-step chemical vapor deposition (CVD) method. The microsheets exhibit hexagonal and trapezoidal shapes, with hexagonal FAPbI3 growing parallel to the substrate and trapezoidal FAPbI3 growing perpendicular to the substrate. The dominant role of single-exciton recombination in the photoluminescence (PL) of these microsheets is observed, especially pronounced at low temperatures, attributed to the relatively large exciton binding energies of the samples. Calculations reveal exciton binding energies as high as 110.8 meV for hexagonal and 133.3 meV for trapezoidal FAPbI3 single-crystal microsheets, attributed to reduced rotational freedom of the formamidinium (FA) ions. Further investigation into low-temperature phase transitions indicates lower transition temperatures (around 100 K) for these microsheets, suggesting reduced FA ion rotational freedom and consequently higher exciton binding energies.

Oriented Epitaxial Growth of Mixed-Dimensional van der Waals Heterostructures with One-Dimensional (1D) Bi2S3 Nanowires and Two-Dimensional (2D) WS2 Monolayers for Performance-Enhanced Photodetectors.

The synthesis of mixed-dimensional van der Waals heterostructures with controlled alignment by chemical vapor deposition (CVD) technique remains a big challenge due to the complex epitaxial growth mechanism. Herein, we report the epitaxial growth of mixed-dimensional Bi2S3/WS2 heterostructures by a two-step CVD method. Bi2S3 crystals grown on 2D WS2 monolayers exhibit 1D feature with the preferred orientation, indicating a strong epitaxial growth behavior at the 1D/2D interface. Furthermore, the heterostructure was carefully characterized by transmission electron microscopy, which reveals the preferential growth of Bi2S3 nanowires along the zigzag edge of WS2 monolayers. The experimental results are also consistent with the theoretical calculations by DFT, where the preferred orientation possesses minimal surface energy. The strong interaction between Bi2S3 and WS2 enables efficient charge transfer of photogenerated carriers at the heterointerface, which leads to a largely improved light harvesting capability with the highest responsivity of ∼48.1 AW-1 and detectivity of ∼5.9 × 1012 Jones.

High-Performance Broadband Image Sensing Photodetector Based on MnTe/WS2 van der Waals Epitaxial Heterostructures.

Two-dimensional transition metal dichalcogenide (TMDC) heterostructure is receiving considerable attention due to its novel electronic, optoelectronic, and spintronic devices with design-oriented and functional features. However, direct design and synthesis of high-quality TMDC/MnTe heterostructures remain difficult, which severely impede further investigations of semiconductor/magnetic semiconductor devices. Herein, the synthesis of high-quality vertically stacked WS2/MnTe heterostructures is realized via a two-step chemical vapor deposition method. Raman, photoluminescence, and scanning transmission electron microscopy characterizations reveal the high-quality and atomically sharp interfaces of the WS2/MnTe heterostructure. WS2/MnTe-based van der Waals field effect transistors demonstrate high rectification behavior with rectification ratio up to 106, as well as a typical p-n electrical transport characteristic. Notably, the fabricated WS2/MnTe photodetector exhibits sensitive and broadband photoresponse ranging from UV to NIR with a maximum responsivity of 1.2 × 103 A/W, a high external quantum efficiency of 2.7 × 105%, and fast photoresponse time of ∼50 ms. Moreover, WS2/MnTe heterostructure photodetectors possess a broadband image sensing capability at room temperature, suggesting potential applications in next-generation high-performance and broadband image sensing photodetectors.

Two-step growth of β-Ga2O3 on c-plane sapphire using MOCVD for solar-blind photodetector

In this work, a two-step metal organic chemical vapor deposition (MOCVD) method was applied for growing β-Ga2O3 film on c-plane sapphire. Optimized buffer layer growth temperature (T B) was found at 700 °C and the β-Ga2O3 film with full width at half maximum (FWHM) of 0.66° was achieved. A metal−semiconductor−metal (MSM) solar-blind photodetector (PD) was fabricated based on the β-Ga2O3 film. Ultrahigh responsivity of 1422 A/W @ 254 nm and photo-to-dark current ratio (PDCR) of 106 at 10 V bias were obtained. The detectivity of 2.5 × 1015 Jones proved that the photodetector has outstanding performance in detecting weak signals. Moreover, the photodetector exhibited superior wavelength selectivity with rejection ratio (R 250 nm/R 400 nm) of 105. These results indicate that the two-step method is a promising approach for preparation of high-quality β-Ga2O3 films for high-performance solar-blind photodetectors.

Vertical heterostructure of graphite-MoS2 for gas sensing.

2D materials, given their form-factor, high surface-to-volume ratio, and chemical functionality have immense use in sensor design. Engineering 2D heterostructures can result in robust combinations of desirable properties but sensor design methodologies require careful considerations about material properties and orientation to maximize sensor response. This study introduces a sensor approach that combines the excellent electrical transport and transduction properties of graphite film with chemical reactivity derived from the edge sites of semiconducting molybdenum disulfide (MoS2) through a two-step chemical vapour deposition method. The resulting vertical heterostructure shows potential for high-performance hybrid chemiresistors for gas sensing. This architecture offers active sensing edge sites across the MoS2 flakes. We detail the growth of vertically oriented MoS2 over a nanoscale graphite film (NGF) cross-section, enhancing the adsorption of analytes such as NO2, NH3, and water vapor. Raman spectroscopy, density functional theory calculations and scanning probe methods elucidate the influence of chemical doping by distinguishing the role of MoS2 edge sites relative to the basal plane. High-resolution imaging techniques confirm the controlled growth of highly crystalline hybrid structures. The MoS2/NGF hybrid structure exhibits exceptional chemiresistive responses at both room and elevated temperatures compared to bare graphitic layers. Quantitative analysis reveals that the sensitivity of this hybrid sensor surpasses other 2D material hybrids, particularly in parts per billion concentrations.

(Invited) Self-Powered Image Sensors Based on Vertical Van Der Waals 2H/1T' -MoTe2 Homojunctions

Image sensors can convert optical images into electrical images, which play important roles in industrial application fields and emerging applications, including digital cameras, biomedical imaging, and machine visions. [1, 2] Compared to traditional three-dimensional rigid materials, two-dimensional (2D) materials with excellent electrical, optoelectrical, and mechanical properties exhibit great potentials in novel image sensors. This work aims to synthesize large scale van der Waals 1T’/2H MoTe2 homojunctions and fabricate image sensors for self-powered imaging functions. Firstly, a two-step chemical vapor deposition (CVD) method is adopted to synthesize the 1T’/2H MoTe2 homojunctions via tellurization process of molybdenum film. Secondly, the structural design of the self-powered photodetector based on 2H MoTe2 with asymmetric electrodes and the corresponding optoelectrical properties are investigated. [3] Thirdly, an image sensor consisting of a self-powered MoTe2 photodetectors array is constructed and exhibits excellent imaging functions with zero power electrical consumption. This work will open a new route to build image systems based on 2D materials, showing great opportunities in digital image systems with ultralow power consumption and excellent imaging functions. Acknowledgement This work was fully supported by a grant from the National Natural Science Foundation of China (Project No. 52022003).

High-performance multilayer WSe2/SnS2 p–n heterojunction photodetectors by two step confined space chemical vapor deposition

Two-dimensional (2D) p–n heterojunctions have attracted great attention due to their outstanding properties in electronic and optoelectronic devices, especially in photodetectors. Various types of heterojunctions have been constituted by mechanical exfoliation and stacking. However, achieving controlled growth of heterojunction structures remains a tremendous challenge. Here, we employed a two-step KI-assisted confined-space chemical vapor deposition method to prepare multilayer WSe2/SnS2 p–n heterojunctions. Optical characterization results revealed that the prepared WSe2/SnS2 vertical heterostructures have clear interfaces as well as vertical heterostructures. The electrical and optoelectronic properties were investigated by constructing the corresponding heterojunction devices, which exhibited good rectification characteristics and obtained a high detectivity of 7.85 × 1012 Jones and a photoresponse of 227.3 A W−1 under visible light irradiation, as well as a fast rise/fall time of 166/440 μs. These remarkable performances are likely attributed to the ultra-low dark current generated in the depletion region at the junction and the high direct tunneling current during illumination. This work demonstrates the value of multilayer WSe2/SnS2 heterojunctions for applications in high-performance photodetectors.

Band type engineering using different stacking configurations of anisotropic and isotropic monolayer transition metal dichalcogenides

This work demonstrates the band-type engineering and the detailed charge transport mechanism upon visible light illumination for various configurations of vertically stacked monolayers of MoS2-ReS2 grown by a two-step chemical vapour deposition method. In order to understand the stacking order of both materials has a direct impact on the band alignment arrangements, we investigate the optical properties of both ReS2–MoS2 stacking configurations using micro-photoluminescence and interestingly observed the change in the band alignment upon changing the stacking order (ReS2–MoS2 and MoS2–ReS2). The formation of the vertically stacked heterostructure is further validated by observing its morphology by HR-TEM. The MoS2 on top of ReS2 yielded Type II and ReS2 on top of MoS2 yielded type I band alignment. The fabricated photodetector exhibits responsivities of 152 A W−1 for pristine ReS2, 72 A W−1 for MoS2 on top, and 400 A W−1 for ReS2 on top respectively for visible light illumination of 554 nm suggesting that the stacking configuration of the monolayer TMDs play a vital role in the performance of the optoelectronic properties. The detailed study of such configurations of vertically stacked 2D heterostructure is essential to better understand the optimal configuration for the development of highly responsive photodetectors.

(Invited) Design of Perovskites for High-Performance Electronics and Optoelectronics

The success of lead halide perovskites as active materials in optoelectronic technologies is often associated with the unique properties of their optical excitations. Specifically, the photo-generated excitons in perovskites are responsible for the large absorption coefficients close to their band gap, reaching as high as 105 cm-1. Due to the advent of nanotechnology, perovskite materials can be readily fabricated into nanoscale configurations with different dimensionalities, and more importantly, the corresponding excitonic effects could be further enhanced. On the other hand, at length scales down to the nanoscale regime, surface features as well as quantum confinement effects, become dominant in regulating the advanced material properties of perovskite materials. [1, 2] In recent years, our group focused on the synthesis and characterization of metal halide perovskites with different forms, ranging from bulk film, microplate, and nanosheet, to nanowire. The corresponding physical properties and device applications were also systematically studied based on their widely tunable dimensionality, morphologies, and compositions.For perovskite bulk films, surface defects and bulk structural order significantly affect device performance. Through optimizing processing techniques, self-assemble quasi-2D perovskite films with graded phase distribution were successfully prepared by our group. [3] Gradient type-II band alignments along the out-of-plane direction of perovskites with spontaneous separation of photo-generated electrons and holes are obtained, which is further employed to construct self-powered vertical-structure photodetectors for the first time. Without any driving voltage, the device exhibited impressive performance with responsivity up to 444 mA/W and ultrashort response time down to 52 µs. In addition, to probe the intrinsic material properties of crystalline perovskites, freestanding MAPbI3 nanosheets, and lead-free Cs3Sb2I9 microplates were fabricated by two-step chemical vapor deposition method, in which excellent optoelectronic performance (e.g., responsively of MAPbI3 nanosheet is measured to be 40 A/W) together with ultra-fast response speed (down to 58 µs) and superior thermal stability were obtained. [4, 5]For nanostructured perovskites, understanding the dimensional features and their impact on the materials and devices is becoming increasingly important. For the first time, we reported the direct vapor-liquid-solid growth of single-crystalline all-inorganic lead halide perovskite (i.e., CsPbX3; X = Cl, Br, or I) NWs using Sn as catalyst seeds. [6] These NWs exhibited high-performance photodetection with the responsivity exceeding 4489 A/W and detectivity over 7.9 × 1012 Jones toward the visible light regime. Field-effect transistors based on individual CsPbX3 NWs were also fabricated to show the impressive carrier mobility of 3.05 cm2/Vs, being higher than other all-inorganic perovskite devices. After that, the Au catalyst seeds were used to avoid the Sn-related impurity doping, by which lower dark current and higher detectivity were achieved in Au-seeded CsPbI3 NWs. Besides, the realization of high-mobility CsPbBr3 NW devices is reported via a simple surface charge transfer doping strategy. [7] After MoO3 decoration and device fabrication, the hole mobility of CsPbBr3/MoO3 core-shell NW device is significantly enhanced to 23.3 cm2/Vs. All these results provide important guidelines for the further improvement of these perovskite nanostructures for practical utilization.

Cobalt Phosphate-Modified (GaN)1–x(ZnO)x/GaN Branched Nanowire Array Photoanodes for Enhanced Photoelectrochemical Performance

Photoelectrochemical water splitting based on suitable catalysts has attracted wide attention as a promising strategy to utilize solar energy to produce clean and renewable hydrogen fuel. Herein, we reported cobalt phosphate-modified (GaN)1–x(ZnO)x/GaN nanowire arrays on a high-temperature conductive GaN substrate toward enhanced photoelectrochemical water splitting by a facile two-step Au-assisted chemical vapor deposition method. The highly conductive Si-doped GaN substrate is designed to serve as a current collector and epitaxial substrate to grow GaN nanowires at high temperature. Meanwhile, high density of branched (GaN)1–x(ZnO)x nanowires with a tunable band gap, strong visible-light absorption, and high catalytic activity is deposited on the surface of GaN nanowires to act as active components to harvest light toward the oxygen evolution reaction. Such multi-junction heterostructures effectively enhance wide spectral utilization and light absorption and simultaneously accelerate the separation of electrons and holes and the transfer of photogenerated electrons from (GaN)1–x(ZnO)x nanowires to the GaN substrate collector and Pt electrode. The photocurrent density of the (GaN)1–x(ZnO)x/GaN nanowire array photoanode can reach 37.5 μA cm–2 at 1.23 V vs reversible hydrogen electrode and could be further enhanced to 186 μA cm–2 by modifying the cobalt phosphate cocatalyst, showing promising potential in clean hydrogen production.

The plasticity of synaptic memristor based on 2D-MoS2 thin film prepared in large-scale by a PLD-assisted CVD method

Two-dimensional MoS2 film was directly grown via a two-step chemical vapour deposition (CVD) method in a large-scale on a phosphorus doped silicon substrate, where a precursor of Mo was Pre-deposited by pulsed laser deposition (PLD) to ensure the homogeneity of Mo source. The film was confirmed to be dense, uniform and continuous nanocrystalline of MoS2 by the characterization of XRD, Raman, SEM, TEM and XPS. Then MoS2 film was employed as the channel materials to fabricate the memristor with lateral structure of Cu/MoS2/Cu. Consequently, the devices show stable nonvolatile resistance switching characteristics, such as low operating voltages, repeated cyclic sweeps (600 times) without decrease and long retention time (>104 s). Such mechanism of resistive switching can be attributable to the space-charge limited conduction dominated by defects. Importantly, a variety of biological synaptic functions, such as synaptic potentiation, depression and repetitive learning processes, are effectively simulated. Furthermore, the relationships between the resistance state and the parameters of pulse sequence were explored based on MoS2 thin film memristors. This research provides a feasible method for manufacturing MoS2 based on memristor, which is used for information and computing applications in neural morphological devices.

Crystalline multilayer graphene nanoflakes synthesized by catalytic chemical vapor deposition using reduced nanostructured hematite as catalyst precursor and 1,2-dichlorobenzene and benzylamine mixture as carbon source

Carbon solubility in the catalyst for the growth of graphene-type nanomaterials is an essential issue for their large-scale fabrication. Iron compounds are fundamental catalysts for the synthesis of carbon materials. The carbon solubility of ferrite (α-Fe) is ∼0.1% at 700 °C, which is enough to promote the growth of graphene nanomaterials. In this work, we synthesized crystalline multilayer graphene flakes (MLGFs) by a two-step catalytic chemical vapor deposition method using an α-Fe catalyst derived from the reduction of nanostructured hematite at 950 °C under an Ar–H2 environment. In the first step, nitrogen-doped multi-walled carbon nanotubes were synthesized by a benzylamine-ferrocene mixture at 850 °C and oxidized at 620 °C in air. In the second step, the reduced nanostructured hematite was exposed to a mixture of a 1:9 benzylamine-1,2- dichlorobenzene at 950 °C. Raman spectroscopy confirmed the presence of MLGFs with a G band and a significant 2D band intensity. The results of the thermogravimetric analysis show that the maximum oxidation rate was at 643.5 °C. Additional functional density calculations were performed considering graphene growth on the oriented (110) α-Fe surface. This work reasserts the possibility of using alternative carbon sources for graphene growth.

(Invited) From Bulk to Nanostructured Perovskites

The dimensionality of semiconductors has a crucial role in determining their properties. Recently, metal halide perovskites have been demonstrated with many exciting applications, attracting wide attention to further their development for advanced optoelectronics, such as photovoltaics, photodetectors, light-emitting diodes, and lasers. At length scales down to nanoscale regimes, surface features as well as quantum confinement effects become dominant in regulating the material properties of perovskite materials. In past years, our group focus on the synthesis and characterization of metal halide perovskites with different configurations, ranging from bulk films, microplates, nanosheets, to nanowires. The corresponding physical properties and device applications were also systematically studied based on their widely tunable dimensionality, morphologies, and compositions.Specifically, for perovskite bulk films, surface defects and bulk structural order significantly affect their device performance. Through optimizing processing techniques, self-assembled quasi-2D perovskite films with graded phase distribution were successfully prepared. Gradient type-II band alignments along the out-of-plane direction of perovskites with spontaneous separation of photo-generated electrons and holes are obtained, which is later employed to construct self-powered vertical-structure photodetectors for the first time. Without any driving voltage, the device exhibited impressive performance with the responsivity up to 444 mA/W and ultrashort response time down to 52 µs. In addition, to assess the intrinsic material properties of crystalline perovskites, freestanding MAPbI3 nanosheets and lead-free Cs3Sb2I9 microplates were fabricated by two-step chemical vapor deposition method, in which excellent optoelectronic performance (e.g., responsively of MAPbI3 nanosheet is measured to be 40 A/W) together with ultra-fast response speed (down to 58 µs) and superior thermal stability were obtained.For nanostructured perovskites, understanding the dimensional features and their impact on the materials and devices is becoming increasingly important. Lately, we reported the direct vapor-liquid-solid growth of single-crystalline all-inorganic lead halide perovskite (i.e., CsPbX3; X = Cl, Br, or I) NWs. These NWs exhibited high-performance photodetection with the responsivity exceeding 4489 A/W and detectivity over 7.9 × 1012 Jones toward the visible light regime. Field-effect transistors based on individual CsPbX3 NWs were also fabricated to show the impressive carrier mobility of 3.05 cm2/Vs, being higher than other all-inorganic perovskite devices. Besides, the realization of high-mobility CsPbBr3 NW devices is reported via a simple surface charge transfer doping strategy. After MoO3 decoration and device fabrication, the hole mobility of CsPbBr3/MoO3 core-shell NW device is significantly enhanced to 23.3 cm2/Vs. All these results provide important guidelines for the further improvement of these perovskite nanostructures for practical utilization.

High performance near-infrared MoTe2/Ge heterojunction photodetector fabricated by direct growth of Ge flake on MoTe2 film substrate

We demonstrated a feasible strategy to fabricate MoTe2/Ge heterojunction by direct growth of Ge flake on a MoTe2 film substrate with a two-step chemical vapor deposition method. A thin transition layer (∼4 nm) mainly composed of polycrystalline germanium at the MoTe2/Ge interface was verified during the Ge flake growth. The MoTe2/Ge heterojunction-based photodetector exhibits both the response speed with a rise/fall time of 7/4 μs and the photoresponsivity and detectivity with 4.87 A W−1 and 5.02 × 1011 Jones under zero bias in the near-infrared regime, respectively. The characteristics of device performance imply its practical applicability as building block for potential near-infrared integrated photonics.

Strong interfacial coupling in vertical WSe2/WS2 heterostructure for high performance photodetection

Two-dimensional van der Waals (vdWs) heterostructures have shown great potential in the field of electronic and optoelectronic applications over the recent decade. However, the controlled preparation of high-quality vdWs heterostructures remains a great challenge ascribing to the incompatibility of different material synthesis processes. Here, we report a two-step chemical vapor deposition method for the growth of high quality WS2 on a WSe2 template with a very low temperature of ∼550 °C, where WO3 nanosheets formed in the first step are further employed as the precursor for synthesis of the top WS2 layer in the second step. Such low growth temperatures in the second step also enable the effective protection of bottom WSe2 during the WS2 growth process and, therefore, result in high-quality WSe2/WS2 heterostructures. Photoluminescence characterizations are further conducted, where significant interlayer exciton emission is observed, indicating strong interlayer coupling in the heterostructure. Based on the obtained WSe2/WS2 heterostructure, dual-channel photosensitive devices were further designed and systematically studied, where high photoresponsivity (3 A/W) as well as fast response speed (<1 ms) are obtained. The developed WO3-assisted growth technique would provide an effective reference for the controlled synthesis of high quality vdWs heterostructures and promote further applications in high-performance optoelectronic devices.

Synthesis and characterization of large-sized monolayer MoS2 nanoflakes by sulfurization of exfoliated MoO3 powder

We demonstrate a novel approach based on sulfurization of MoO3 powder pre-exfoliated on the SiO2/Si substrate to synthesize high crystalline quality and large-sized monolayer MoS2 films uniformly covering the whole substrate. The surface morphology, thickness, crystalline quality, and luminescence mechanism of the MoS2 films were characterized by the optical microscopy, Raman spectroscopy, and photoluminescence spectroscopy. The results reveal that MoS2 films directly synthesized on the bare SiO2/Si substrate by our method have high surface coverage superior over those of one-step chemical vapor deposition (CVD) method and exhibit uniform monolayer with larger lateral length up to ∼120 µm compared to those of the traditional two-step CVD method. Our proposed growth strategy opens up new pathways for CVD synthesis of high crystalline quality and large-sized uniform monolayer MoS2 films and can also be utilized for synthesizing other two-dimensional materials.

Free-standing Si3N4 nanowires@pyrolytic carbon membranes decorated with metal oxide nanoarrays for flexible hybrid supercapacitors

To satisfy the increasing requirement of flexible energy storage devices, various free-standing flexible conductive substrates were developed to construct high-performance supercapacitor electrodes. In this work, free-standing, flexible and conductive Si3N4 nanowires (NWs)@pyrolytic carbon (PyC) membrane, served as nanoscale substrate, was fabricated by a two-step chemical vapor deposition (CVD) method. NiCo2O4 nanosheets (NSs) and Fe2O3 nanorods (NRs) were finally grown on Si3N4NWs@PyC membrane substrates, which were directly served as the positive and negative electrodes, respectively. The membrane electrodes possesss the structural advantages, such as the interwoven network structure of nanowires substrates and array structure of active materials, which guarantee the excellent electrochemical performance of electrodes. Additionally, the flexible solid-state hybrid supercapacitor (HSC) assembled by Si3N4NWs@PyC@NiCo2O4NSs and Si3N4NWs@PyC@Fe2O3NRs, delivered a high energy density of 74.3 Wh kg−1 at a power density of 0.8 kW kg−1, excellent cycling stability (Capacitance retention of 84.6% after 4000 cycles), and good flexibility. Impressively, this work provides a new thinking about the structural design of high-performance supercapacitor electrodes as well as expands the application field of ceramic nanowires macrostructures.

Controllable growth of 2H-1 T′ MoS2/ReS2 heterostructures via chemical vapor deposition

Two-dimensional (2D) 2H-1 T′ heterojunctions based on transition-metal dichalcogenides have attracted great attention due to their special interface structures and novel properties. Understanding the growth mechanism and the interface structures is essential for their controllable growth. In this work, we successfully synthesize lateral and vertical MoS2/ReS2 heterojunctions by using a two-step chemical vapor deposition (CVD) method, and deeply explore the evolution of the structure. It is found that the growth mode of heterojunction undergoes a transition from lateral epitaxy to vertical stacking under a high Re/S ratio during the growth. Furthermore, the interface structure is greatly determined by the edge shape of MoS2 core. Two kinds of structures, with the b-axis of 1 T′ ReS2 shell parallel to the (100) and (010) plane of MoS2 core respectively, are observed. Meanwhile, by comparing with MoS2 monolayer, MoS2/ReS2 heterostructures possess a reduced emission intensity due to the carrier transfer. This work provides a robust and controllable strategy for the synthesis of heterostructures with different phase structures.

Photoemission oscillation in epitaxially grown van der Waals β-In2Se3WS2 heterobilayer bubbles* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51732010 and 51972280), the Natural Science Foundation of Hebei Province, China (Grant No. E2019203233), the Research Program of the College Science & Technology of Hebei Province, China (Grant No. ZD2020121).

Thin films of millimeter-scale continuous monolayer WS2 have been grown on SiO2/Si substrate, followed by the deposition of β-In2Se3 crystals on monolayer WS2 to prepare In2Se3/WS2 van de Waals heterostructures by a two-step chemical vapor deposition (CVD) method. After the growth of In2Se3 at elevated temperatures, high densities of In2Se3/WS2 heterostructure bubbles with monolayer to multilayer β-In2Se3 crystals atop are observed. Fluorescence of the resultant β-In2Se3/WS2 heterostructure is greatly enhanced in intensity upon the formation of bubbles, which are evidenced by the Newton’s rings in optical image owing to constructive and destructive interference. In photoluminescence (PL) mapping images of monolayer β-In2Se3/monolayer WS2 heterobilayer bubble, significant oscillatory behavior of emission intensity is demonstrated due to constructive and destructive interference. However, oscillatory behaviors of peak position are also observed and come from a local heating effect induced by an excitation laser beam. The oscillatory mechanism of PL is further verified by changing the exterior pressure of bubbles placed in a home-made vacuum chamber. In addition, redshifted in peak position and broadening in peak width are observed due to strain effect during decreasing the exterior pressure of bubbles.

Two-Step Chemical Vapor Deposition-Synthesized Lead-Free All-Inorganic Cs3Sb2Br9 Perovskite Microplates for Optoelectronic Applications.

Lead-based halide perovskites (APbX3, where A = organic or inorganic cation, X = Cl, Br, I) are suitable materials for many optoelectronic devices due to their many attractive properties. However, the concern of lead toxicity and the poor ambient and operational stability of the organic cation group greatly limit their practical utilization. Therefore, there has recently been great interest in lead-free, environment-friendly all-inorganic halide perovskites (IHPs). Sb and Sn are common species suggested to replace Pb for Pb-free IHPs. However, the large difference in the melting points of the precursor materials (e.g., CsBr and SbBr3 precursors for Cs3Sb2Br9) makes the chemical vapor deposition (CVD) growth of high-quality Pb-free IHPs a very challenging task. In this work, we developed a two-step CVD method to overcome this challenge and successfully synthesized Pb-free Cs3Sb2Br9 perovskite microplates. Cs3Sb2Br9 microplates ∼25 μm in size with the exciton absorption peak at ∼2.8 eV and a band gap of ∼2.85 eV were obtained. The microplates have a smooth hexagonal morphology and show a large Stokes shift of ∼450 meV and exciton binding energy of ∼200 meV. To demonstrate the applications of these microplates in optoelectronics, simple photoconductive devices were fabricated. These photodetectors exhibit a current on/off ratio of 2.36 × 102, a responsivity of 36.9 mA/W, and a detectivity of 1.0 × 1010 Jones with a fast response of rise and decay time of 61.5 and 24 ms, respectively, upon 450 nm photon irradiation. Finally, the Cs3Sb2Br9 microplates also show good stability in ambient air without encapsulation. These results demonstrate that the 2-step CVD process is an effective approach to synthesize high-quality all-inorganic lead-free Cs3Sb2Br9 perovskite microplates that have the potential for future high-performance optoelectronic device applications.