As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal-semiconductor interface at the source and drain electrodes increases, ...dominating the performance of devices
. Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS
) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors
. However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide
. Recent studies have shown that van der Waals contacts formed by transferred graphene
and metals
on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS
. Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS
is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS
. The contact resistance of the indium/gold electrodes is 3,000 ± 300 ohm micrometres for monolayer MoS
and 800 ± 200 ohm micrometres for few-layered MoS
. These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS
, enabling high-performance field-effect transistors with a mobility of 167 ± 20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220 ± 50 ohm micrometres on ultrathin niobium disulfide (NbS
) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS
) and tungsten diselenide (WSe
) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors.
Metallic transition metal dichalcogenides (TMDs)
are good catalysts for the hydrogen evolution reaction (HER). The overpotential and Tafel slope values of metallic phases and edges
of two-dimensional ...(2D) TMDs approach those of Pt. However, the overall current density of 2D TMD catalysts remains orders of magnitude lower (~10-100 mA cm
) than industrial Pt and Ir electrolysers (>1,000 mA cm
)
. Here, we report the synthesis of the metallic 2H phase of niobium disulfide with additional niobium (2H Nb
S
, where x is ~0.35)
as a HER catalyst with current densities of >5,000 mA cm
at ~420 mV versus a reversible hydrogen electrode. We find the exchange current density at 0 V for 2H Nb
S
to be ~0.8 mA cm
, corresponding to a turnover frequency of ~0.2 s
. We demonstrate an electrolyser based on a 2H Nb
S
cathode that can generate current densities of 1,000 mA cm
. Our theoretical results reveal that 2H Nb
S
with Nb-terminated surface has free energy for hydrogen adsorption that is close to thermoneutral, facilitating HER. Therefore, 2H Nb
S
could be a viable catalyst for practical electrolysers.
Atomically thin tungsten disulfide (WS2), a structural analogue to MoS2, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related ...physics. However, the batch production of layered WS2 is underdeveloped (as compared with that of MoS2) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS2 flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm2. Intriguingly, we show that, with adding minor H2 carrier gas, the shape of monolayer WS2 flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts. Hereby, controlling the initial nucleation state is crucial for synthesizing large-scale single-crystalline flakes. We believe that this work would benefit the controlled growth of high-quality transition metal dichalcogenide, as well as in their future applications in nanoelectronics, optoelectronics, and solar energy conversions.
In-plane and vertically stacked heterostructures of graphene and hexagonal boron nitride (h-BN-G and G/h-BN, respectively) are both recent focuses of graphene research. However, targeted synthesis of ...either heterostructure remains a challenge. Here, via chemical vapour deposition and using benzoic acid precursor, we have achieved the selective growth of h-BN-G and G/h-BN through a temperature-triggered switching reaction. The perfect in-plane h-BN-G is characterized by scanning tunnelling microscopy (STM), showing atomically patched graphene and h-BN with typical zigzag edges. In contrast, the vertical alignment of G/h-BN is confirmed by unique lattice-mismatch-induced moiré patterns in high-resolution STM images, and two sets of aligned selected area electron diffraction spots, both suggesting a van der Waals epitaxial mechanism. The present work demonstrates the chemical designability of growth process for controlled synthesis of graphene and h-BN heterostructures. With practical scalability, high uniformity and quality, our approach will promote the development of graphene-based electronics and optoelectronics.
Direct growth of graphene on traditional glasses is of great importance for various daily life applications. We report herein the catalyst-free atmospheric-pressure chemical vapor deposition approach ...to directly synthesizing large-area, uniform graphene films on solid glasses. The optical transparency and sheet resistance of such kinds of graphene glasses can be readily adjusted together with the experimentally tunable layer thickness of graphene. More significantly, these graphene glasses find a broad range of real applications by enabling the low-cost construction of heating devices, transparent electrodes, photocatalytic plates, and smart windows. With a practical scalability, the present work will stimulate various applications of transparent, electrically and thermally conductive graphene glasses in real-life scenarios.
Catalyst-free and scalable synthesis of graphene on various glass substrates at low temperatures is of paramount significance to numerous applications such as low-cost transparent electronics and ...state-of-the-art displays. However, systematic study within this promising research field has remained scarce thus far. Herein, we report the direct growth of graphene on various glasses using a low-temperature plasma-enhanced chemical vapor deposition method. Such a facile and scalable approach guarantees the growth of uniform, transfer-free graphene films on various glass substrates at a growth temperature range of 400-600 ℃. The morphological, surface wetting, optical, and electrical properties of the obtained graphene can be tailored by controlling the growth parameters. Our uniform and high-quality graphene films directly integrated with low-cost, commonly used glasses show great potential in the fabrication of multi-functional electrodes for versatile applications in solar cells, transparent electronics, and smart windows.
Transferring MoS2 films from growth substrates onto target substrates is a critical issue for their practical applications. Moreover, it remains a great challenge to avoid sample degradation and ...substrate destruction, because the current transfer method inevitably employs a wet chemical etching process. We developed an etching-free transfer method for transferring MoS2 films onto arbitrary substrates by using ultrasonication. Briefly, the collapse of ultrasonication-generated microbubbles at the interface between polymer-coated MoS2 film and substrates induce sufficient force to delaminate the MoS2 films. Using this method, the MoS2 films can be transferred from all substrates （silica, mica, strontium titanate, and sapphire） and retains the original sample morphology and quality. This method guarantees a simple transfer process and allows the reuse of growth substrates, without involving any hazardous etchants. The etching-free transfer method is likely to promote broad applications of MoS2 in photodetectors.
Early transition metals, especially groups IVB-VIB metals, can form stable carbides, which are known to exhibit excellent “noble-metal-like” catalytic activities. We demonstrate herein the ...applications of groups IVB-VIB metals in graphene growth using atmospheric pressure chemical vapor deposition technique. Similar to the extensively studied Cu, Ni, and noble metals, these transition-metal foils facilitate the catalytic growth of single- to few-layer graphene. The most attractive advantage over the existing catalysts is their perfect control of layer thickness and uniformity with highly flexible experimental conditions by in situ converting the dissolved carbons into stable carbides to fully suppress the upward segregation/precipitation effect. The growth performance of graphene on these transition metals can be well explained by the periodic physicochemical properties of elements. Our work has disclosed a new territory of catalysts in the periodic table for graphene growth and is expected to trigger more interest in graphene research.
Chemical vapor deposition （CVD） synthesis of large-domain hexagonal boron nitride （h-BN） with a uniform thickness is very challenging, mainly due to the extremely high nucleation density of this ...material. Herein, we report the successful growth of wafer-scale, high-quality h-BN monolayer films that have large single-crystalline domain sizes, up to -72 μm in edge length, prepared using a folded Cu-foil enclosure. The highly confined growth space and the smooth Cu surface inside the enclosure effectively reduced the precursor feeding rate together and induced a drastic decrease in the nucleation density. The orientation of the as-grown h-BN monolayer was found to be strongly correlated to the crystallographic orientation of the Cu substrate： the Cu （111） face being the best substrate for growing aligned h-BN domains and even single-crystalline monolayers. This is consistent with our density functional theory calculations. The present study offers a practical pathway for growing high-quality h-BN films by deepening our fundamental understanding of the process of their growth by CVD.
Mass production of high-quality graphene with low cost is the footstone for its widespread practical applications. We present herein a self-limited growth approach for producing graphene powders by a ...small-methane-flow chemical vapour deposition process on naturally abundant and industrially widely used diatomite (biosilica) substrates. Distinct from the chemically exfoliated graphene, thus-produced biomorphic graphene is highly crystallized with atomic layer-thickness controllability, structural designability and less noncarbon impurities. In particular, the individual graphene microarchitectures preserve a three-dimensional naturally curved surface morphology of original diatom frustules, effectively overcoming the interlayer stacking and hence giving excellent dispersion performance in fabricating solution-processible electrodes. The graphene films derived from as-made graphene powders, compatible with either rod-coating, or inkjet and roll-to-roll printing techniques, exhibit much higher electrical conductivity (∼110,700 S m
at 80% transmittance) than previously reported solution-based counterparts. This work thus puts forward a practical route for low-cost mass production of various powdery two-dimensional materials.
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