Robotic four-dimensional pixel assembly of van der Waals solids.

Van der Waals (vdW) solids can be engineered with atomically precise vertical composition through the assembly of layered two-dimensional materials1,2. However, the artisanal assembly of structures from micromechanically exfoliated flakes3,4 is not compatible with scalable and rapid manufacturing. Further engineering of vdW solids requires precisely designed and controlled composition over all three spatial dimensions and interlayer rotation. Here, we report a robotic four-dimensional pixel assembly method for manufacturing vdW solids with unprecedented speed, deliberate design, large area and angle control. We used the robotic assembly of prepatterned 'pixels' made from atomically thin two-dimensional components. Wafer-scale two-dimensional material films were grown, patterned through a clean, contact-free process and assembled using engineered adhesive stamps actuated by a high-vacuum robot. We fabricated vdW solids with up to 80 individual layers, consisting of 100 × 100 µm2 areas with predesigned patterned shapes, laterally/vertically programmed composition and controlled interlayer angle. This enabled efficient optical spectroscopic assays of the vdW solids, revealing new excitonic and absorbance layer dependencies in MoS2. Furthermore, we fabricated twisted N-layer assemblies, where we observed atomic reconstruction of twisted four-layer WS2 at high interlayer twist angles of ≥4°. Our method enables the rapid manufacturing of atomically resolved quantum materials, which could help realize the full potential of vdW heterostructures as a platform for novel physics2,5,6 and advanced electronic technologies7,8.

Extremely anisotropic van der Waals thermal conductors.

The densification of integrated circuits requires thermal management strategies and high thermal conductivity materials1-3. Recent innovations include the development of materials with thermal conduction anisotropy, which can remove hotspots along the fast-axis direction and provide thermal insulation along the slow axis4,5. However, most artificially engineered thermal conductors have anisotropy ratios much smaller than those seen in naturally anisotropic materials. Here we report extremely anisotropic thermal conductors based on large-area van der Waals thin films with random interlayer rotations, which produce a room-temperature thermal anisotropy ratio close to 900 in MoS2, one of the highest ever reported. This is enabled by the interlayer rotations that impede the through-plane thermal transport, while the long-range intralayer crystallinity maintains high in-plane thermal conductivity. We measure ultralow thermal conductivities in the through-plane direction for MoS2 (57 ± 3 mW m-1 K-1) and WS2 (41 ± 3 mW m-1 K-1) films, and we quantitatively explain these values using molecular dynamics simulations that reveal one-dimensional glass-like thermal transport. Conversely, the in-plane thermal conductivity in these MoS2 films is close to the single-crystal value. Covering nanofabricated gold electrodes with our anisotropic films prevents overheating of the electrodes and blocks heat from reaching the device surface. Our work establishes interlayer rotation in crystalline layered materials as a new degree of freedom for engineering-directed heat transport in solid-state systems.

Atomically Thin, Optically Isotropic Films with 3D Nanotopography.

Flat optics aims for the on-chip miniaturization of optical systems for high-speed and low-power operation, with integration of thin and lightweight components. Here, we present atomically thin yet optically isotropic films realized by using three-dimensional (3D) topographic reconstruction of anisotropic two-dimensional (2D) films to balance the out-of-plane and in-plane optical responses on the subwavelength scale. We achieve this by conformal growth of monolayer transition metal dichalcogenide (TMD) films on nanodome-structured substrates. The resulting films show an order-of-magnitude increase in the out-of-plane susceptibility for enhanced angular performance, displaying polarization isotropy in the off-axis absorption, as well as improved photoluminescence emission profiles, compared to their flat-film counterparts. We further show that such 3D geometric programming of optical properties is applicable to different TMD materials, offering spectral generalization over for the entire visible range. Our approach presents a powerful platform for advancing the development of atomically thin flat optics with custom-designed light-matter interactions.

SynCells: A 60 × 60 µm2 Electronic Platform with Remote Actuation for Sensing Applications in Constrained Environments.

Autonomous electronic microsystems smaller than the diameter of a human hair (<100 µm) are promising for sensing in confined spaces such as microfluidic channels or the human body. However, they are difficult to implement due to fabrication challenges and limited power budget. Here we present a 60 × 60 µm electronic microsystem platform, or SynCell, that overcomes these issues by leveraging the integration capabilities of two-dimensional material circuits and the low power consumption of passive germanium timers, memory-like chemical sensors, and magnetic pads. In a proof-of-concept experiment, we magnetically positioned SynCells in a microfluidic channel to detect putrescine. After we extracted them from the channel, we successfully read out the timer and sensor signal, the latter of which can be amplified by an onboard transistor circuit. The concepts developed here will be applicable to microsystems targeting a variety of applications from microfluidic sensing to biomedical research.

Wafer-scale synthesis of monolayer two-dimensional porphyrin polymers for hybrid superlattices.

The large-scale synthesis of high-quality thin films with extensive tunability derived from molecular building blocks will advance the development of artificial solids with designed functionalities. We report the synthesis of two-dimensional (2D) porphyrin polymer films with wafer-scale homogeneity in the ultimate limit of monolayer thickness by growing films at a sharp pentane/water interface, which allows the fabrication of their hybrid superlattices. Laminar assembly polymerization of porphyrin monomers could form monolayers of metal-organic frameworks with Cu2+ linkers or covalent organic frameworks with terephthalaldehyde linkers. Both the lattice structures and optical properties of these 2D films were directly controlled by the molecular monomers and polymerization chemistries. The 2D polymers were used to fabricate arrays of hybrid superlattices with molybdenum disulfide that could be used in electrical capacitors.

Two-Dimensional Material Tunnel Barrier for Josephson Junctions and Superconducting Qubits.

Quantum computing based on superconducting qubits requires the understanding and control of the materials, device architecture, and operation. However, the materials for the central circuit element, the Josephson junction, have mostly been focused on using the AlOx tunnel barrier. Here, we demonstrate Josephson junctions and superconducting qubits employing two-dimensional materials as the tunnel barrier. We batch-fabricate and design the critical Josephson current of these devices via layer-by-layer stacking N layers of MoS2 on the large scale. Based on such junctions, MoS2 transmon qubits are engineered and characterized in a bulk superconducting microwave resonator for the first time. Our work allows Josephson junctions to access the diverse material properties of two-dimensional materials that include a wide range of electrical and magnetic properties, which can be used to study the effects of different material properties in superconducting qubits and to engineer novel quantum circuit elements in the future.

Morphology-Retained Photoconversion Reaction of Anthracene Single Crystal: A New Approach for Organic Heterostructures.

Morphology-retained solid-state photoconversion of anthracene (AN) to 9,10-anthraquinone (PC-ANQ) and dipara-anthracene (PC-DPA) was accomplished by irradiating mercury lamp light to plate-shaped AN single crystal in oxygen and argon atmosphere, respectively. The photoconverted crystals retained the original plate shape morphology of the starting AN crystal, whereas the emission profile and crystal structure were significantly changed. The electrical conductivity of PC-ANQ crystal is 5 orders of magnitude greater than that of the starting AN crystal, whereas the PC-DPA crystal exhibits a decreased conductivity. The AN/PC-ANQ/PC-DPA heterostructures with smooth interface were successfully obtained by inducing the photoconversion only at the desired area.

Highly reproducible alkali metal doping system for organic crystals through enhanced diffusion of alkali metal by secondary thermal activation.

In this paper, we report an efficient alkali metal doping system for organic single crystals. Our system employs an enhanced diffusion method for the introduction of alkali metal into organic single crystals by controlling the sample temperature to induce secondary thermal activation. Using this system, we achieved intercalation of potassium into picene single crystals with closed packed crystal structures. Using optical microscopy and Raman spectroscopy, we confirmed that the resulting samples were uniformly doped and became K2picene single crystal, while only parts of the crystal are doped and transformed into K2picene without secondary thermal activation. Moreover, using a customized electrical measurement system, the insulator-to-semiconductor transition of picene single crystals upon doping was confirmed by in situ electrical conductivity and ex situ temperature-dependent resistivity measurements. X-ray diffraction studies showed that potassium atoms were intercalated between molecular layers of picene, and doped samples did not show any KH- nor KOH-related peaks, indicating that picene molecules are retained without structural decomposition. During recent decades, tremendous efforts have been exerted to develop high-performance organic semiconductors and superconductors, whereas as little attention has been devoted to doped organic crystals. Our method will enable efficient alkali metal doping of organic crystals and will be a resource for future systematic studies on the electrical property changes of these organic crystals upon doping.

Coherent, atomically thin transition-metal dichalcogenide superlattices with engineered strain.

Epitaxy forms the basis of modern electronics and optoelectronics. We report coherent atomically thin superlattices in which different transition metal dichalcogenide monolayers-despite large lattice mismatches-are repeated and laterally integrated without dislocations within the monolayer plane. Grown by an omnidirectional epitaxy, these superlattices display fully matched lattice constants across heterointerfaces while maintaining an isotropic lattice structure and triangular symmetry. This strong epitaxial strain is precisely engineered via the nanoscale supercell dimensions, thereby enabling broad tuning of the optical properties and producing photoluminescence peak shifts as large as 250 millielectron volts. We present theoretical models to explain this coherent growth and the energetic interplay governing the ripple formation in these strained monolayers. Such coherent superlattices provide building blocks with targeted functionalities at the atomically thin limit.

Unique Crystallization of Fullerenes: Fullerene Flowers.

Solution-phase crystallization of fullerene molecules strongly depends on the types of solvent and their ratios because solvent molecules are easily included in the crystal lattice and distort its structure. The C70 (solute)-mesitylene (solvent) system yields crystals with various morphologies and structures, such as cubes, tubes, and imperfect rods. Herein, using C60 and C70 dissolved in mesitylene, we present a novel way to grow unique flower-shaped crystals with six symmetric petals. The different solubility of C60 and C70 in mesitylene promotes nucleation of C70 with sixfold symmetry in the early stage, which is followed by co-crystallization of both C60 and C70 molecules, leading to lateral petal growth. Based on the growth mechanism, we obtained more complex fullerene crystals, such as multi-deck flowers and tube-flower complexes, by changing the sequence and parameters of crystallization.

Layer-Confined Excitonic Insulating Phase in Ultrathin Ta2NiSe5 Crystals.

Atomically thin nanosheets, as recently realized using van der Waals layered materials, offer a versatile platform for studying the stability and tunability of the correlated electron phases in the reduced dimension. Here, we investigate a thickness-dependent excitonic insulating (EI) phase on a layered ternary chalcogenide Ta2NiSe5. Using Raman spectroscopy, scanning tunneling spectroscopy, and in-plane transport measurements, we found no significant changes in crystalline and electronic structures as well as disorder strength in ultrathin Ta2NiSe5 crystals with a thickness down to five layers. The transition temperature, Tc, of ultrathin Ta2NiSe5 is reduced from its bulk value by ΔTc/Tc(bulk) ≈ -9%, which strongly contrasts the case of 1T-TiSe2, another excitonic insulator candidate, showing an increase of Tc by ΔTc/Tc(bulk) ≈ +30%. This difference is attributed to the dominance of interband Coulomb interaction over electron-phonon interaction and its zero-ordering wave vector due to the direct band gap structure of Ta2NiSe5. The out-of-plane correlating length of the EI phase is estimated to have monolayer thickness, suggesting that the EI phase in Ta2NiSe5 is highly layer-confined and in the strong coupling limit.

Value-added Synthesis of Graphene: Recycling Industrial Carbon Waste into Electrodes for High-Performance Electronic Devices.

We have developed a simple, scalable, transfer-free, ecologically sustainable, value-added method to convert inexpensive coal tar pitch to patterned graphene films directly on device substrates. The method, which does not require an additional transfer process, enables direct growth of graphene films on device substrates in large area. To demonstrate the practical applications of the graphene films, we used the patterned graphene grown on a dielectric substrate directly as electrodes of bottom-contact pentacene field-effect transistors (max. field effect mobility ~0.36 cm(2)·V(-1)·s(-1)), without using any physical transfer process. This use of a chemical waste product as a solid carbon source instead of commonly used explosive hydrocarbon gas sources for graphene synthesis has the dual benefits of converting the waste to a valuable product, and reducing pollution.

Germanium Silicon Alloy Anode Material Capable of Tunable Overpotential by Nanoscale Si Segregation.

We developed the novel electrode that enables fine control of overpotential by exploiting surface segregation that is the enrichment of one component at the surface of binary alloy. To realize this approach, we controlled the proportion of Si with low Li diffusivity at the surface by annealing the SiGe nanowire in H2 environment at various temperatures. The resulting SiGe nanowires annealed at 850 °C exhibited high reversible capacity (>1031 mA·h·g(-1)), and long cycle life (400 cycles) with high capacity retention (89.0%) at 0.2 C. This superior battery performance is attributed to the remaining unlithiated part acting as support frame to prevent pulverization of anode material, which results from the fine-tuning of overpotential by controlling the degree of Si segregation.

Crystallization-induced properties from morphology-controlled organic crystals.

During the past two decades, many materials chemists have focused on the development of organic molecules that can serve as the basis of cost-effective and flexible electronic, optical, and energy conversion devices. Among the potential candidate molecules, metal-free or metal-containing conjugated organic molecules offer high-order electronic conjugation levels that can directly support fast charge carrier transport, rapid optoelectric responses, and reliable exciton manipulation. Early studies of these molecules focused on the design and synthesis of organic unit molecules that exhibit active electrical and optical properties when produced in the form of thin film devices. Since then, researchers have worked to enhance the properties upon crystallization of the unit molecules as single crystals provide higher carrier mobilities and exciton recombination yields. Most recently, researchers have conducted in-depth studies to understand how crystallization induces property changes, especially those that depend on specific crystal surfaces. The different properties that depend on the crystal facets have been of particular interest. Most unit molecules have anisotropic structures, and therefore produce crystals with several unique crystal facets with dissimilar molecular arrangements. These structural differences would also lead to diverse electrical conductance, optical absorption/emission, and even chemical interaction properties depending on the crystal facet investigated. To study the effects of crystallization and crystal facet-dependent property changes, researchers must grow or synthesize crystals of highly conjugated molecules that have both a variety of morphologies and high crystallinity. Morphologically well-defined organic crystals, that form structures such as wires, rods, disks, and cubes, provide objects that researchers can use to evaluate these material properties. Such structures typically occur as single crystals with well-developed facets with dissimilar molecular arrangements. Recently, researchers have proposed several approaches for the vapor and solution phase synthesis of high quality organic crystals with various morphologies. In this Account, we focus on methodologies for the synthesis of various organic- and metal-containing highly conjugated molecular crystals. We also examine the new optical and chemical properties of these materials. In addition, we introduce recent experimental results demonstrating that high crystallinity and specific molecular arrangements lead to crystallization-induced property changes. We believe that the understanding of the crystallization-induced property changes in organic crystals will provide both fundamental knowledge of the chemical processes occurring at various interfaces and opportunities for researchers to take advantage of crystallization-induced property changes in the development of high-performance organic devices.

Patternable large-scale molybdenium disulfide atomic layers grown by gold-assisted chemical vapor deposition.

A novel way to grow MoS2 on a large scale with uniformity and in desired patterns is developed. We use Au film as a catalyst on which [Mo(CO)6 ] vapor decomposes to form a Mo-Au surface alloy that is an ideal Mo reservoir for the growth of atomic layers of MoS2 . Upon exposure to H2 S, this surface alloy transforms into a few layers of MoS2 , which can be isolated and transferred on an arbitrary substrate. By simply patterning Au catalyst film by conventional lithographic techniques, MoS2 atomic layers in desired patterns can be fabricated.

Vertical crystallization of C60 nanowires by solvent vapor annealing process.

We report that C60 molecules are spontaneously crystallized into vertical nanowires by the solvent vapor annealing (SVA) process. C60 molecules have been known to be assembled into wire-like crystals by simply dropping and drying C60 solutions in m-xylene on a solid substrate. By the drop-drying process, C60 nanowires have been mostly grown laterally on a solid substrate, as the major force applied to the droplet during the drying process is parallel to the substrate. On the other hand, the SVA process seems to provide an ideal environment under which the direction of the dominant drying force of a droplet becomes vertical. When a thermally evaporated C60 film is exposed to m-xylene solvent vapor under controlled SVA environments at room temperature, C60 molecules are found to be crystallized into vertical nanowires. The effect of solvent vapor pressure on the vertical growth of C60 nanowire is examined by comparative studies using mesitylene and 1,3-dichlorobenzene. The versatility of the SVA process for the growth of vertical organic nanostructures is further demonstrated by the successful formations of vertically grown C60 2D disks and 5,7,12,14-pentacenetetrone anisotropic crystals by employing carbon tetrachloride and toluene solvent vapors, respectively.

Synthesis of a p-type semiconducting phenothiazine exfoliatable layered crystal.

Phenothiazine (PTZ) crystals are grown by a physical vapor transport method in a horizontal tube furnace. The resulting disk-type PTZ single crystals have a layered structure, which can be mechanically exfoliated into stacked individual layers to exhibit various colors depending on the thickness. The PTZ single-crystal field-effect transistor (FET) devices exhibit a p-type semiconducting property with 3.03 × 10(-6) S/cm electrical conductivity, 1.15 × 10(-4) cm(2)/(V s) carrier mobility, and a 10(4) on/off ratio.

Copper-vapor-assisted chemical vapor deposition for high-quality and metal-free single-layer graphene on amorphous SiO2 substrate.

We report that high-quality single-layer graphene (SLG) has been successfully synthesized directly on various dielectric substrates including amorphous SiO2/Si by a Cu-vapor-assisted chemical vapor deposition (CVD) process. The Cu vapors produced by the sublimation of Cu foil that is suspended above target substrates without physical contact catalyze the pyrolysis of methane gas and assist nucleation of graphene on the substrates. Raman spectra and mapping images reveal that the graphene formed on a SiO2/Si substrate is almost defect-free and homogeneous single layer. The overall quality of graphene grown by Cu-vapor-assisted CVD is comparable to that of the graphene grown by regular metal-catalyzed CVD on a Cu foil. While Cu vapor induces the nucleation and growth of SLG on an amorphous substrate, the resulting SLG is confirmed to be Cu-free by synchrotron X-ray photoelectron spectroscopy. The SLG grown by Cu-vapor-assisted CVD is fabricated into field effect transistor devices without transfer steps that are generally required when SLG is grown by regular CVD process on metal catalyst substrates. This method has overcome two important hurdles previously present when the catalyst-free CVD process is used for the growth of SLG on fused quartz and hexagonal boron nitride substrates, that is, high degree of structural defects and limited size of resulting graphene, respectively.

A novel role of three dimensional graphene foam to prevent heater failure during boiling.

We report a novel boiling heat transfer (NBHT) in reduced graphene oxide (RGO) suspended in water (RGO colloid) near critical heat flux (CHF), which is traditionally the dangerous limitation of nucleate boiling heat transfer because of heater failure. When the heat flux reaches the maximum value (CHF) in RGO colloid pool boiling, the wall temperature increases gradually and slowly with an almost constant heat flux, contrary to the rapid wall temperature increase found during water pool boiling. The gained time by NBHT would provide the safer margin of the heat transfer and the amazing impact on the thermal system as the first report of graphene application. In addition, the CHF and boiling heat transfer performance also increase. This novel boiling phenomenon can effectively prevent heater failure because of the role played by the self-assembled three-dimensional foam-like graphene network (SFG).