Large-area Synthesis Research Articles

Synthesis of Wafer-Scale Monolayer Pyrenyl Graphdiyne on Ultrathin Hexagonal Boron Nitride for Multibit Optoelectronic Memory.

Graphdiyne is a new two-dimensional carbon allotrope with many attractive properties and has been widely used in various applications. However, the synthesis of large-area, high-quality, and ultrathin (especially monolayer) graphdiyne and its analogues remains a challenge, hindering its application in optoelectronic devices. Here, a wafer-scale monolayer pyrenyl graphdiyne (Pyr-GDY) film is obtained on hexagonal boron nitride (hBN) via a van der Waals epitaxial strategy, and top-floating-gated multibit nonvolatile optoelectronic memory based on Pyr-GDY/hBN/graphene is constructed, using Pyr-GDY as a photoresponsive top-floating gate. Benefiting from the excellent charge trapping capability and strong absorption of the graphdiyne film, as well as the top-floating-gated structure and the ultrathin hBN film used in the device, the optoelectronic memory exhibits high storage performance and robust reliability. A huge difference in the current between the programmed and erased states (>26 μA μm-1 at Vds = 0.1 V) and a prolonged retention time (>105 s) enable the device to achieve multibit storage, for which eight and nine distinct storage levels (3-bit) are obtained by applying periodic gate voltages and optical pulses in the programming and erasing processes, respectively. This work provides an important step toward realizing versatile graphdiyne-based optoelectronic devices in the future.

Ultrasensitive Phototransistor Based on WSe2-MoS2 van der Waals Heterojunction.

Band engineering using the van der Waals heterostructure of two-dimensional materials allows for the realization of high-performance optoelectronic devices by providing an ultrathin and uniform PN junction with sharp band edges. In this study, a highly sensitive photodetector based on the van der Waals heterostructure of WSe2 and MoS2 was developed. The MoS2 was utilized as the channel for a phototransistor, whereas the WSe2-MoS2 PN junction in the out-of-plane orientation was utilized as a charge transfer layer. The vertical built-in electric field in the PN junction separated the photogenerated carriers, thus leading to a high photoconductive gain of 106. The proposed phototransistor exhibited an excellent performance, namely, a high photoresponsivity of 2700 A/W, specific detectivity of 5 × 1011 Jones, and response time of 17 ms. The proposed scheme in conjunction with the large-area synthesis technology of two-dimensional materials contributes significantly to practical photodetector applications.

Application-Oriented Growth of a Molybdenum Disulfide (MoS2) Single Layer by Means of Parametrically Optimized Chemical Vapor Deposition.

In the 2D material framework, molybdenum disulfide (MoS2) was originally studied as an archetypical transition metal dichalcogenide (TMD) material. The controlled synthesis of large-area and high-crystalline MoS2 remains a challenge for distinct practical applications from electronics to electrocatalysis. Among the proposed methods, chemical vapor deposition (CVD) is a promising way for synthesizing high-quality MoS2 from isolated domains to a continuous film because of its high flexibility. Herein, we report on a systematic study of the effects of growth pressure, temperature, time, and vertical height between the molybdenum trioxide (MoO3) source and the substrate during the CVD process that influence the morphology, domain size, and uniformity of thickness with controlled parameters over a large scale. The substrate was pretreated with perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) seed molecule that promoted the layer growth of MoS2. Further, we characterized the as-grown MoS2 morphologies, layer quality, and physical properties by employing scanning electron microscopy (SEM), Raman spectroscopy, and photoluminescence (PL). Our experimental findings demonstrate the effectiveness and versatility of the CVD approach to synthesize MoS2 for various target applications.

Ultrafast nucleation and growth of high-quality monolayer MoSe2 crystals via vapor-liquid-solid mechanism

The controlled production of two-dimensional atomically thin transition metal dichalcogenides (TMDs) is fundamentally important for their device applications. However, the synthesis of large-area and high-quality TMD monolayers remains a challenge due to the lack of sufficient understanding of growth mechanisms, especially for the chemical vapor deposition (CVD). Here we report molten-salt assisted CVD growth of highly crystalline MoSe2 monolayers via a novel vapor-liquid-solid (VLS) mechanism. Our results show that the growth rate of the VLS-grown monolayer MoSe2 is about 40 times faster than that of MoSe2 grown via the vapor-solid (VS) mechanism, which makes the fabrication of 100 μm domains for ∼2 min and a uniform monolayer film within 5 min. The ultrafast growth of monolayer MoSe2 crystals benefits from the synergic effect of one-dimensional VLS growth and two-dimensional VS edge expansion. Moreover, these MoSe2 monolayers exhibit high crystal quality and enhanced photoluminescence due to efficient Se-vacancy repairing by the doping of halogen atoms. These findings provide a new understanding of MoSe2 growth and open up an opportunity for the rapid synthesis of high-quality TMD monolayers and heterostructures.

Large-area synthesis of a semiconducting silver monolayer via intercalation of epitaxial graphene

Two-dimensional monolayers consisting of a single element have attracted considerable interest due to their intriguing properties, which can be fundamentally different from the bulk counterparts. However, their large-scale synthesis often remains challenging owing to the nonlayered nature of the respective bulk crystal structures. In this Rapid Communication we show that the noble metal silver can be confined into the monolayer limit via intercalation between silicon carbide and epitaxial buffer layer graphene. Using angle-resolved photoelectron spectroscopy we reveal the formation of a silver-related valence band whose dispersion can be described by a simple, triangular-lattice tight-binding model. Interestingly, the synthesized silver monolayer is semiconducting as opposed to the prototype $sp$ bulk metal. The intercalation process further yields an $n$-type doped quasi-free-standing graphene monolayer, thereby realizing a two-dimensional metal/semiconductor heterostructure. Our results demonstrate the potential of epitaxial graphene on silicon carbide as a functional platform for the wafer-scale synthesis of monoelemental monolayers with unique attributes.

Semiconductor to metal transition in two-dimensional gold and its van der Waals heterostack with graphene

The synthesis of two-dimensional (2D) transition metals has attracted growing attention for both fundamental and application-oriented investigations, such as 2D magnetism, nanoplasmonics and non-linear optics. However, the large-area synthesis of this class of materials in a single-layer form poses non-trivial difficulties. Here we present the synthesis of a large-area 2D gold layer, stabilized in between silicon carbide and monolayer graphene. We show that the 2D-Au ML is a semiconductor with the valence band maximum 50 meV below the Fermi level. The graphene and gold layers are largely non-interacting, thereby defining a class of van der Waals heterostructure. The 2D-Au bands, exhibit a 225 meV spin-orbit splitting along the overline {{mathrm{Gamma }}{mathrm{K}}} direction, making it appealing for spin-related applications. By tuning the amount of gold at the SiC/graphene interface, we induce a semiconductor to metal transition in the 2D-Au, which has not yet been observed and hosts great interest for fundamental physics.

Large area few-layer TMD film growths and their applications

Research on 2D materials is one of the core themes of modern condensed matter physics. Prompted by the experimental isolation of graphene, much attention has been given to the unique optical, electronic, and structural properties of these materials. In the past few years, semiconducting transition metal dichalcogenides (TMDs) have attracted increasing interest due to properties such as direct band gaps and intrinsically broken inversion symmetry. Practical utilization of these properties demands large-area synthesis. While films of graphene have been by now synthesized on the order of square meters, analogous achievements are difficult for TMDs given the complexity of their growth kinetics. This article provides an overview of methods used to synthesize films of mono- and few-layer TMDs, comparing spatial and time scales for the different growth strategies. A special emphasis is placed on the unique applications enabled by such large-scale realization, in fields such as electronics and optics.

Large-area synthesis of van der Waals two-dimensional material Nb3I8 and its infrared detection applications

The two-dimensional (2D) inorganic material Nb3I8, which can be separated into atomic planes, was synthesized through a vapor transport reaction between niobium and iodine. High-purity and cm scale large Nb3I8 crystals were obtained by controlling the growth temperature (600–700 °C) and the stoichiometric ratio between niobium and iodine (using excess I2 condition, niobium: iodine = 3:9). The semiconductor characteristics were measured by the change of electrical conductivity with temperature (increased conductivity with temperature in the range of 100 K–300 K), and have a band gap of ∼1 eV (∼1200 nm) through Infrared (IR)-visible light absorption analysis. Based on these characteristics, the device characteristics of detecting IR near 1200 nm region were verified. Besides few-layers of Nb3I8 was acquired by the mechanical exfoliation method, and experimentally observed by atomic force microscopy (AFM). This 2D material is expected to have an important role in developing another 2D materials and in new nano-optoelectronic devices using it.

First-Principles Predictions and Synthesis of B50C2 by Chemical Vapor Deposition

Density functional theory predictions have been combined with the microwave-plasma chemical vapor deposition technique to explore metastable synthesis of boron-rich boron-carbide materials. A thin film synthesis of high-hardness (up to 37 GPa) B50C2 via chemical vapor deposition was achieved. Characterization of the experimental crystal structure matches well with a new theoretical model structure, with carbon atoms inserted into the boron icosahedra and 2b sites in a α-tetragonal B52 base structure. Previously reported metallic B50C2 structures with carbons inserted only into the 2b or 4c sites are found to be dynamically unstable. The newly predicted structure is insulating and dynamically stable, with a computed hardness value and electrical properties in excellent agreement with the experiment. The present study thus validates the density functional theory calculations of stable crystal structures in boron-rich boron-carbide system and provides a pathway for large-area synthesis of novel materials by the chemical vapor deposition method.

Molecular dynamics simulation of graphene sinking during chemical vapor deposition growth on semi-molten Cu substrate

Copper foil is the most promising catalyst for the synthesis of large-area, high-quality monolayer graphene. Experimentally, it has been found that the Cu substrate is semi-molten at graphene growth temperatures. In this study, based on a self-developed C–Cu empirical potential and density functional theory (DFT) methods, we performed systematic molecular dynamics simulations to explore the stability of graphene nanostructures, i.e., carbon nanoclusters and graphene nanoribbons, on semi-molten Cu substrates. Many atomic details observed in the classical MD simulations agree well with those seen in DFT-MD simulations, confirming the high accuracy of the C–Cu potential. Depending on the size of the graphene island, two different sunken-modes are observed: (i) graphene island sinks into the first layer of the metal substrate and (ii) many metal atoms surround the graphene island. Further study reveals that the sinking graphene leads to the unidirectional alignment and seamless stitching of the graphene islands, which explains the growth of large single-crystal graphene on Cu foil. This study deepens our physical insights into the CVD growth of graphene on semi-molten Cu substrate with multiple experimental mysteries well explained and provides theoretic references for the controlled synthesis of large-area single-crystalline monolayer graphene.

The controlled large-area synthesis of two dimensional metals

The rise of nanotechnology has been propelled by low dimensional metals. Albeit the long perceived importance, synthesis of freestanding metallic nanomembranes, or the so-called 2D metals, however has been restricted to simple metals with a very limited in-plane size (<10 μm). In this work, we developed a low-cost method to synthesize 2D metals through polymer surface buckling enabled exfoliation. The 2D metals so obtained could be as chemically complex as high entropy alloys while possessing in-plane dimensions at the scale of bulk metals (>1 cm). With our approach, we successfully synthesized a variety of 2D metals, such as 2D high entropy alloy and 2D metallic glass, with controllable geometries and morphologies. Moreover, our approach can be readily extended to non-metals and composites, thereby opening a large window to the fabrication of a wide range of 2D materials of technologic importance which have never been reported before.

Improving Formation Conditions and Properties of hBN Nanosheets Through BaF2-assisted Polymer Derived Ceramics (PDCs) Technique.

Hexagonal boron nitrite (hBN) is an attractive material for many applications such as in electronics as a complement to graphene, in anti-oxidation coatings, light emitters, etc. However, the synthesis of high-quality hBN at cost-effective conditions is still a great challenge. Thus, this work reports on the synthesis of large-area and crystalline hBN nanosheets via the modified polymer derived ceramics (PDCs) process. The addition of both the BaF2 and Li3N, as melting-point reduction and crystallization agents, respectively, led to the production of hBN powders with excellent physicochemical properties at relatively low temperatures and atmospheric pressure conditions. For instance, XRD, Raman, and XPS data revealed improved crystallinity and quality at a decreased formation temperature of 1200 °C upon the addition of 5 wt% of BaF2. Moreover, morphological determination illustrated the formation of multi-layered nanocrystalline and well-defined shaped hBN powders with crystal sizes of 2.74–8.41 ± 0.71 µm in diameter. Despite the compromised thermal stability, as shown by the ease of oxidation at high temperatures, this work paves way for the production of large-scale and high-quality hBN crystals at a relatively low temperature and atmospheric pressure conditions.

Anatomy of On-Surface Synthesized Boroxine Two-Dimensional Polymers.

Synthetic two-dimensional polymers (2DPs) obtained from well-defined monomers via bottom-up fabrication strategies are promising materials that can extend the realm of inorganic 2D materials. The on-surface synthesis of such 2DPs is particularly popular, however the pathway complexity in the growth of such films formed on solid surfaces is poorly understood. In this contribution, we present a straightforward experimental protocol which allows the synthesis of large-area, defect-free 2DPs based on boroxine linkages at room temperature. We focus on unravelling the multiple pathways available to the polymerizing system for the spatial extension of the covalent bonds. Besides the anticipated 2DP, the system can evolve into self-assembled monolayers of partially fused monodisperse reaction products that are difficult to isolate by conventional synthetic methods or remain in the monomeric state. The access to each pathway can be controlled via monomer concentration and the choice of the solvent. Most importantly, the unpolymerized systems do not evolve into the corresponding 2DP upon annealing, indicating the presence of strong kinetic traps. Using high-resolution scanning tunneling microscopy, we show reversibility in the polymerization process where the attachment and the detachment of monomers to 2DP crystallites could be monitored as a function of time. Finally, we show that the way the 2DP grows depends on the choice of the solvent. Using UV-vis absorption and emission spectroscopy, we reveal that the dominant pathway for 2DP growth is via in-plane self-condensation of the monomers, whereas in the case of an aprotic solvent, the favored growth mode is via π stacking of the monomers.

High-performance transparent conductive pyrolyzed carbon (Py-C) ultrathin film

This study proposes a substrate independent, large-area synthesis strategy for pyrolyzed carbon (Py-C) ultrathin films directly on various target substrates. The Py-C film has opto-electro-mechanical properties comparable to those of graphene. We demonstrated a highly flexible pixelated display composed of the Py-C film. The Py-C film showed remarkable performance as a protective layer against Cu oxidation and the chemical etching of ITO.

Deeply Exploring Anisotropic Evolution toward Large-Scale Growth of Monolayer ReS2.

Among large numbers of transition metal dichalcogenides (TMDCs), monolayer rhenium disulfide (ReS2) is of particular interest due to its unique structural anisotropy, which opens up unprecedented opportunities in dichroic atomical electronics. Understanding the domain structure and controlling the anisotropic evolution of ReS2 during the growth is considered critical for increasing the domain size toward a large-scale growth of monolayer ReS2. Herein, by employing angle-resolved Raman spectroscopy, we reveal that the hexagonal ReS2 domain is constructed by six well-defined subdomains with each b-axis parallel to the diagonal of the hexagon. By further combining the first-principles calculations and the transmission electron microscopy (TEM) characterization, a dislocation-involved anisotropic evolution is proposed to explain the formation of the domain structures and understand the limitation of the domain size. Based on these findings, growth rates of different crystal planes are well controlled to enlarge the domain size, and moreover, single-crystal domains with a triangle shape are obtained. With the improved domain size, large-scale uniform, strictly monolayer ReS2 films are grown further. Scalable field-effect transistor (FET) arrays are constructed, which show good electrical performances comparable or even superior to that of the single domains reported at room temperature. This work not only sheds light on comprehending the novel growth mechanism of ReS2 but also offers a robust and controllable strategy for the synthesis of large-area and high-quality two-dimensional materials with low structural symmetry.

Scanning tunneling microscopic investigations for studying conformational change of underlying Cu(111) and Ni(111) during graphene growth

Demonstration of graphene growth on metal substrates using chemical vapor deposition method (CVD) facilitated the large-area synthesis of graphene, which extended the graphene research from the fundamental investigation with a small graphene flake to the various practical applications. Spectroscopic studies using 13C-labeled carbon precursor first suggested two different growth mechanisms of graphene on Ni and Cu substrates during the conventional CVD process, ‘segregation growth’ and ‘surface growth’ for Ni and Cu substrates, respectively. However, the growth mechanism of graphene is not simply understood. For example, the surface science studies have suggested the Ni2C conversion mechanism or the direct surface growth even on Ni surface. Herein, the conformational changes of underlying metal substrate during graphene growth on Ni(111) and Cu(111) are presented, which are highly affected by the different types of growth mechanisms. We believe that our results may widen the knowledge to understand the graphene growth mechanisms on Ni and Cu. Also, the influence of graphene-metal interaction on the electronic structures has been elucidated for graphene grown on Ni and Cu substrates by scanning tunneling spectroscopic studies combined with the theoretical calculation.

Trickle Flow Aided Atomic Layer Deposition (ALD) Strategy for Ultrathin Molybdenum Disulfide (MoS2) Synthesis.

The synthesis of large-area and high-quality two-dimensional (2D) MoS2 is undoubtedly a significant challenge till now. In this work, an effective strategy for achieving 2D MoS2 with enlarged grain size by Ni-foam-based trickle flow atomic layer deposition (ALD) was suggested, by which MoS2 grain sizes up to 420 nm (monolayer sample) and 400 nm (five-layer sample) were obtained under the covering of 1 mm-thick Ni foam with a 2 mm gap from the substrate at 460 °C. In terms of specific ALD experiments, the Ni foam with a certain thickness placed on top of the substrate made the original precursor flow into a trickle-fluidization source flow, which decreased the nucleation density effectively. Thus, MoS2 with enlarged grain sizes were obtained based on the typical ALD mechanism of large-scale vertical growth under the action of steric hindrance after a planar parallel growth around the crystal nucleus. In addition, the Ni foam also achieved a stable temperature field by enhancing the heat transfer around the substrate and thus improved its crystallinity.

Structural, Spectroscopic, and Excitonic Dynamic Characterization in Atomically Thin Yb3+‐Doped MoS2, Fabricated by Femtosecond Pulsed Laser Deposition

AbstractThe large area deposition and synthesis of 10 mm × 10 mm atomically thin Yb3+‐doped MoS2 films by femtosecond pulsed laser deposition on a silica glass optical platform for device applications are demonstrated for the first time. The presence of Yb3+‐ion doping is confirmed using photoluminescence (PL), X‐ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The Yb3+‐doped MoS2 films, when excited with a 976 nm laser, exhibit room temperature PL with a peak at 1002 nm. The XPS and Raman spectroscopic analyses of the Yb3+‐doped and undoped films show that the deposited films are a mixture of 2H‐ and 1T‐MoS2 after postdeposition annealing at 500 °C. The density functional theory analysis shows that the 1T phase is metastable by +77 kJ (≈0.8 eV) mol‐1, when compared with the 2H state at 0 K. Ultrafast transient nonlinear optical spectroscopic measurements prove that the saturable absorption of undoped MoS2 is significantly modified after Yb3+‐ion doping, by displaying dopant‐host structure charge transfer. The complex transient absorption line shape shows a combination of bleach (negative) signals at the A (670 nm) and B (630 nm) exciton energies, and a strong induced absorption below the A exciton level. The results presented herein provide critical insight in designing novel rare‐earth‐ion doped 2D materials and devices.

(Invited) Two-Dimensional Layered Materials Toward Phase-Engineered Hybrid Films

2D materials have attracted much attention because of frontier electronic materials due to its superior electronic transport properties and mechanical flexibility in the future, making it a potential material for high performance and wearable electronics. Graphene is a typical 2D materials with high carrier mobility; however, it still cannot be applied in transistor due to the lack of bandgap. A new type of 2D semiconducting materials called transition metal dichalcogenides (TMDCs), which are layered structure with the strong in-plane bonding and weak out-of-plane interactions similar to graphite, have been intensively studied. Recent studies have predicted exceptional physical properties upon reduced dimensionality attracting lots of attention due to the versatile physical chemical behaviors. Nevertheless, the synthesis and the study of the fundamental physical properties of TMDs are still in early stages. The lack of a large-area and reliable synthesis method restrict exploring all the potential applications of the TMDs. Chemical vapor deposition (CVD) is a traditional approach for the growth of TMDs; nevertheless, the high growth temperature is a major drawback for its to be applied in flexible electronics. In this talk, an inductively coupled plasma (ICP) was used to synthesize Transition Metal Dichalcogenides (TMDs) through a plasma-assisted selenization process of metal oxide (MOx) at a low temperature, as low as 250 °C. Compared to other CVD processes the use of ICP facilitates the decomposition of the precursors at lower temperatures; therefore, the temperature required for the formation of TMDs can be drastically reduced. WSe2 was chosen as a model material system due to its technological importance as a p-type inorganic semiconductor with an excellent hole mobility. Large-area synthesis of WSe2 on polyimide (30 x 40 cm2) flexible substrates and 8-inch silicon wafers with good uniformity was demonstrated at the formation temperature of 250 °C as confirmed by Raman and X-ray Photoelectron (XPS) spectroscopy. Furthermore, by controlling different H2/N2 ratios, hybrid WOx/WSe2 films can be formed at the formation temperature of 250 ºC as shown by TEM and confirmed by XPS. The applications including (1) water splitting, (2) gas sensors and (3) photodetectors will be reported.

Vertically Stacked CVD-Grown 2D Heterostructure for Wafer-Scale Electronics

This paper demonstrates, for the first time, wafer-scale graphene/MoS2 heterostructures prepared by chemical vapor deposition (CVD) and their application in vertical transistors and logic gates. A CVD-grown bulk MoS2 layer is utilized as the vertical channel, whereas CVD-grown monolayer graphene is used as the tunable work-function electrode. The short vertical channel of the transistor is formed by sandwiching bulk MoS2 between the bottom indium tin oxide (ITO, drain electrode) and the top graphene (source electrode). The electron injection barriers at the graphene-MoS2 junction and ITO-MoS2 junction are modulated effectively through variation of the Schottky barrier height and its effective barrier width, respectively, because of the work-function tunability of the graphene electrode. The resulting vertical transistor with the CVD-grown MoS2/graphene heterostructure exhibits a current density exceeding 7 A/cm2, a subthreshold swing of 410 mV/dec, and an on-off current ratio exceeding 103. The large-area synthesis, transfer, and patterning processes of both semiconducting MoS2 and metallic graphene facilitate construction of a wafer-scale array of transistors and logic gates such as NOT, NAND, and NOR.