Xingxing Zhang

Noble‐transition‐metal dichalcogenide has emerged as a unique 2D material for fundamental studies and various applications. A comprehensive review is provided regarding the unique structure and novel physical properties for a wide range of applications. In addition, current progress and latest advances in the device application are summarized.

Abstract

Emerging classes of 2D noble‐transition‐metal dichalcogenides (NTMDs) stand out for their unique structure and novel physical properties in recent years. With the nearly full occupation of the d orbitals, 2D NTMDs are expected to be more attractive due to the unique interlayer vibrational behaviors and largely tunable electronic structures compared to most transition metal dichalcogenide semiconductors. The novel properties of 2D NTMDs have stimulated various applications in electronics, optoelectronics, catalysis, and sensors. Here, the latest development of 2D NTMDs are reviewed from the perspective of structure characterization, preparation, and application. Based on the recent research, the conclusions and outlook for these rising 2D NTMDs are presented.

A comprehensive review of thermal transport of various emerging 2D semiconductors is provided here. The phonon‐related phenomenon is discussed alongside issues encountered in various applications based on them. Furthermore, a thorough understanding of phonon transport physics in 2D semiconductors to inform the thermal management of next‐generation nanoelectronic devices is provided, and strategies for controlling heat energy transport and conversion are also considered.

Abstract

The discovery of graphene has stimulated the search for and investigations into other 2D materials because of the rich physics and unusual properties exhibited by many of these layered materials. Transition metal dichalcogenides (TMDs), black phosphorus, and SnSe among many others, have emerged to show highly tunable physical and chemical properties that can be exploited in a whole host of promising applications. Alongside the novel electronic and optical properties of such 2D semiconductors, their thermal transport properties have also attracted substantial attention. Here, a comprehensive review of the unique thermal transport properties of various emerging 2D semiconductors is provided, including TMDs, black‐ and blue‐phosphorene among others, and the different mechanisms underlying their thermal conductivity characteristics. The focus is placed on the phonon‐related phenomena and issues encountered in various applications based on 2D semiconductor materials and their heterostructures, including thermoelectric power generation and electron–phonon coupling effect in photoelectric and thermal transistor devices. A thorough understanding of phonon transport physics in 2D semiconductor materials to inform thermal management of next‐generation nanoelectronic devices is comprehensively presented along with strategies for controlling heat energy transport and conversion.

For the first time, 2D ultrathin Te flakes (5 nm) are successfully realized by hydrogen‐assisted chemical vapor deposition method. The density functional theory calculations and experiments confirm that two volatile intermediates increase the vapor pressure of the source and promote the reaction. Impressively, the Te‐flake‐based phototransistor shows giant gate‐dependent photoresponse.

Abstract

Tellurium (Te), as an elementary material, has attracted intense attention due to its potentially novel properties. However, it is still a great challenge to realize high‐quality 2D Te due to its helical chain structure. Here, ultrathin Te flakes (5 nm) are synthesized via hydrogen‐assisted chemical vapor deposition method. The density functional theory calculations and experiments confirm the growth mechanism, which can be ascribed to the formation of volatile intermediates increasing vapor pressure of the source and promoting the reaction. Impressively, the Te flake‐based transistor shows high on/off ratio ≈10⁴, ultralow off‐state current ≈8 × 10⁻¹³ A, as well as a negligible hysteresis due to reducing thermally activated defects at 80 K. Moreover, Te‐flake‐based phototransistor demonstrates giant gate‐dependent photoresponse: when gate voltage varies from −70 to 70 V, I _on/I _off is increased by ≈40‐fold. The hydrogen‐assisted strategy may provide a new approach for synthesizing other high quality 2D elementary materials.

The use of liquid exfoliated 2D WS₂ and MoS₂ as hole‐transporting layers (HTLs) in ultrahigh efficiency organic solar cells is reported. WS₂ yields cells with higher power conversion efficiency (PCE), fill‐factor, and short‐circuit current than MoS₂ and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate). When WS₂ is introduced as HTL in PBDB‐T‐2F:Y6:PC₇₁BM organic solar cells, a maximum PCE value of 17% is achieved.

Abstract

The application of liquid‐exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene‐based organic solar cells is reported. It is shown that solution processing of few‐layer WS₂ or MoS₂ suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS₂ are found to exhibit higher uniformity on ITO than those of MoS₂ and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short‐circuit current (J _SC), and lower series resistance than devices based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) and MoS₂. Cells based on the ternary bulk‐heterojunction PBDB‐T‐2F:Y6:PC₇₁BM with WS₂ as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open‐circuit voltage of 0.84 V, and a J _SC of 26 mA cm⁻². Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS₂‐based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high‐efficiency organic photovoltaics.

Large‐size 1T‐VSe₂ monolayers are successfully produced at high yield by electrochemical exfoliation of bulk crystal. To guard against air‐induced degradation, thiol molecules are introduced to passivate the VSe₂ flakes, allowing the observation of robust room‐temperature ferromagnetism in monolayer VSe₂.

Abstract

Among van der Waals layered ferromagnets, monolayer vanadium diselenide (VSe₂) stands out due to its robust ferromagnetism. However, the exfoliation of monolayer VSe₂ is challenging, not least because the monolayer flake is extremely unstable in air. Using an electrochemical exfoliation approach with organic cations as the intercalants, monolayer 1T‐VSe₂ flakes are successfully obtained from the bulk crystal at high yield. Thiol molecules are further introduced onto the VSe₂ surface to passivate the exfoliated flakes, which improves the air stability of the flakes for subsequent characterizations. Room‐temperature ferromagnetism is confirmed on the exfoliated 2D VSe₂ flakes using a superconducting quantum interference device (SQUID), X‐ray magnetic circular dichroism (XMCD), and magnetic force microscopy (MFM), where the monolayer flake displays the strongest ferromagnetic properties. Se vacancies, which can be ubiquitous in such materials, also contribute to the ferromagnetism of VSe₂, although density functional theory (DFT) calculations show that such effect can be minimized by physisorbed oxygen molecules or covalently bound thiol molecules.

Comprehensive knowledge of and recent progress in controlled growth of single‐crystal graphene films using chemical vapor deposition are presented. The main methods for enlarging graphene domain size and reducing graphene grain‐boundary density are classified into single‐seed and multiseed approaches, based on which detailed synthesis strategies, related mechanisms, key parameters, and limiting factors are summarized.

Abstract

Grain boundaries produced during material synthesis affect both the intrinsic properties of materials and their potential for high‐end applications. This effect is commonly observed in graphene film grown using chemical vapor deposition and therefore caused intense interest in controlled growth of grain‐boundary‐free graphene single crystals in the past ten years. The main methods for enlarging graphene domain size and reducing graphene grain boundary density are classified into single‐seed and multiseed approaches, wherein reduction of nucleation density and alignment of nucleation orientation are respectively realized in the nucleation stage. On this basis, detailed synthesis strategies, corresponding mechanisms, and key parameters in the representative methods of these two approaches are separately reviewed, with the aim of providing comprehensive knowledge and a snapshot of the latest status of controlled growth of single‐crystal graphene films. Finally, perspectives on opportunities and challenges in synthesizing large‐area single‐crystal graphene films are discussed.

A lightweight three‐dimensional (3D) Cu nanowire network with a lithiophilic phosphidation gradient along the cross‐section is used as the current collector for a Li‐metal anode. The gradient structure and the formed Li₃P‐rich surface with good ionic/electrical conductivity guide the stable deposition of Li ions inside the 3D structure, improving the Li mass loading and the energy density of the anode.

Abstract

Lithium metal anodes with high energy density are important for further development of next‐generation batteries. However, inhomogeneous Li deposition and dendrite growth hinder their practical utilization. 3D current collectors are widely investigated to suppress dendrite growth, but they usually occupy a large volume and increase the weight of the system, hence decreasing the energy density. Additionally, the nonuniform distribution of Li ions results in low utilization of the porous structure. A lightweight, 3D Cu nanowire current collector with a phosphidation gradient is reported to balance the lithiophilicity with conductivity of the electrode. The phosphide gradient with good lithiophilicity and high ionic conductivity enables dense nucleation of Li and its steady deposition in the porous structure, realizing a high pore utilization. Specifically, the homogenous deposition of Li leads to the formation of an oriented texture on the electrode surface at high capacities. A high mass loading (≈44 wt%) of Li with a capacity of 3 mAh cm⁻² and a high average Coulombic efficiency of 97.3% are achieved. A lifespan of 300 h in a symmetrical cell is obtained at 2 mA cm⁻², implying great potential to stabilize lithium metal.

Scanning probe lithography is used to pattern 2D semiconductors. It is a “clean” process for etching that does not leave photoresist residues at the edge of the patterned monolayers, while quantifying the edge quality by edge recombination velocity. The possibility of passivating the edges by chemical treatments, as the edges are exposed without any residual coating, is also highlighted.

Abstract

Scanning probe lithography is used to directly pattern monolayer transition metal dichalcogenides (TMDs) without the use of a sacrificial resist. Using an atomic‐force microscope, a negatively biased tip is brought close to the TMD surface. By inducing a water bridge between the tip and the TMD surface, controllable oxidation is achieved at the sub‐100 nm resolution. The oxidized flake is then submerged into water for selective oxide removal which leads to controllable patterning. In addition, by changing the oxidation time, thickness tunable patterning of multilayer TMDs is demonstrated. This resist‐less process results in exposed edges, overcoming a barrier in traditional resist‐based lithography and dry etch where polymeric byproduct layers are often formed at the edges. By patterning monolayers into geometric patterns of different dimensions and measuring the effective carrier lifetime, the non‐radiative recombination velocity due to edge defects is extracted. Using this patterning technique, it is shown that selenide TMDs exhibit lower edge recombination velocity as compared to sulfide TMDs. The utility of scanning probe lithography towards understanding material‐dependent edge recombination losses without significantly normalizing edge behaviors due to heavy defect generation, while allowing for eventual exploration of edge passivation schemes is highlighted, which is of profound interest for nanoscale electronics and optoelectronics.

The orientation of low‐dimensional crystal‐structural films significantly affects the performance of photoelectric devices. A method of seed screening is developed to control the orientation of low‐dimensional crystal‐structural films, such as 1D Sb₂Se₃ and 2D SnSe. Applying this method to a Sb₂Se₃ film, a record efficiency of 7.62% is achieved in TiO₂/Sb₂Se₃ solar cells.

Abstract

The orientation of low‐dimensional crystal‐structural (LDCS) films significantly affects the performance of photoelectric devices, particularly in vertical conducting devices such as solar cells and light‐emitting diodes. According to film growth theory, the initial seeds determine the final orientation of the film. Ruled by the minimum energy principle, lying (chains or layers parallel to the substrate) seeds bonding with the substrate through van der Waals forces are easier to form than standing (chains or layers perpendicular to the substrate) seeds bonding with the substrate by a covalent bond. Utilizing high substrate temperature to re‐evaporate the lying seeds and preserve the standing seeds, the orientation of 1D crystal‐structural Sb₂Se₃ is successfully controlled. Guided by this seed screening model, highly [211]‐ and [221]‐oriented Sb₂Se₃ films on an inert TiO₂ substrate are obtained; consequently, a record efficiency of 7.62% in TiO₂/Sb₂Se₃ solar cells is achieved. This universal model of seed screening provides an effective method for orientation control of other LDCS films.

A two‐dimensional striated lattice with sub‐nanometer one‐dimensional channels of Pd₂Se₃ is created in situ by annealing few‐layered PdSe₂ two‐dimensional films at 400 °C. Annular dark‐field scanning transmission electron microscopy reveals the atomic structure and phase transformation of the striated lattice and density functional theory predicts the one‐dimensional channels act as localized conduction pathways within the system.

Abstract

2D crystals are typically uniform and periodic in‐plane with stacked sheet‐like structure in the out‐of‐plane direction. Breaking the in‐plane 2D symmetry by creating unique lattice structures offers anisotropic electronic and optical responses that have potential in nanoelectronics. However, creating nanoscale‐modulated anisotropic 2D lattices is challenging and is mostly done using top‐down lithographic methods with ≈10 nm resolution. A phase transformation mechanism for creating 2D striated lattice systems is revealed, where controlled thermal annealing induces Se loss in few‐layered PdSe₂ and leads to 1D sub‐nm etched channels in Pd₂Se₃ bilayers. These striated 2D crystals cannot be described by a typical unit cells of 1–2 Å for crystals, but rather long range nanoscale periodicity in each three directions. The 1D channels give rise to localized conduction states, which have no bulk layered counterpart or monolayer form. These results show how the known family of 2D crystals can be extended beyond those that exist as bulk layered van der Waals crystals by exploiting phase transformations by elemental depletion in binary systems.

Nature Electronics, Published online: 17 September 2019; doi:10.1038/s41928-019-0301-7

The formation of a chemically stable mixture of tungsten microparticles and liquid metal (LM) with 2–3 times enhanced thermal conductivity (60 W m⁻¹ K⁻¹) and easy‐to‐apply paste consistency is enabled through blending in an oxygen‐rich environment. In situ imaging reveals how the adhesion of nanoscale gallium oxide flakes on the LM‐phobic particles enables their wetting and paste formation.

Abstract

Modern microelectronics and emerging technologies such as wearable devices and soft robotics require conformable and thermally conductive thermal interface materials to improve their performance and longevity. Gallium‐based liquid metals (LMs) are promising candidates for these applications yet are limited by their moderate thermal conductivity, difficulty in surface‐spreading, and pump‐out issues. Incorporation of metallic particles into the LM can address these problems, but observed alloying processes shift the LM melting point and lead to undesirable formation of additional surface roughness. Here, these problems are addressed by introducing a mixture of tungsten microparticles dispersed within a LM matrix (LM‐W) that exhibits two‐ to threefold enhanced thermal conductivity (62 ± 2.28 W m⁻¹ K⁻¹ for gallium and 57 ± 2.08 W m⁻¹ K⁻¹ for EGaInSn at a 40% filler volume mixing ratio) and liquid‐to‐paste transition for better surface application. It is shown that the formation of a nanometer‐scale LM oxide in oxygen‐rich environments allows highly nonwetting tungsten particles to mix into LMs. Using in situ imaging and particle dipping experimentation within a focused ion beam and scanning electron microscopy system, the oxide‐assisted mechanism behind this wetting process is revealed. Furthermore, since tungsten does not undergo room‐temperature alloying with gallium, it is shown that LM‐W remains a chemically stable mixture.

The transition‐metal diphosphide WP₂ is a type‐II Weyl semimetal candidate. The anisotropy of its ac‐plane resistivity, which mainly arises from the scattering rate anisotropy, increases sharply at temperature T ≤ 100 K without phase transitions and can be tuned by magnetic fields. The broken inversion symmetry in WP₂ is identified by combining linearly polarized Raman spectroscopy and first‐principle calculations.

Abstract

A transition metal diphosphide, WP₂, is a candidate for type‐II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP₂, spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP₂ electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle‐resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first‐principle studies of WP₂ are reported. The energies of the observed Raman‐active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first‐principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP₂. Moreover, the measured ratio (R_c /R_a ) between the crystalline c‐axis and a‐axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in‐plane electrical resistivity anisotropy among the reported type‐II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first‐principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the R_c /R_a of WP₂ at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so‐far‐strongest and magnetic‐field‐tunable electrical resistivity anisotropy found in WP₂ can serve as a degree of freedom for tuning the electrical properties of type‐II WSMs, which paves the way for the development of novel electronic applications based on type‐II WSMs.

Proton irradiation of bulk transition‐metal dichalcogenides leads to the blistering of atomically thin domes filled with hydrogen. The domes stud the crystal surface and locally turn the dark bulk material into an efficient light emitter. They can be produced with ordered positions and sizes tunable from the nanometer to the micrometer scale, with important prospects for so far unattainable applications.

Abstract

At the few‐atom‐thick limit, transition‐metal dichalcogenides (TMDs) exhibit strongly interconnected structural and optoelectronic properties. The possibility to tailor the latter by controlling the former is expected to have a great impact on applied and fundamental research. As shown here, proton irradiation deeply affects the surface morphology of bulk TMD crystals. Protons penetrate the top layer, resulting in the production and progressive accumulation of molecular hydrogen in the first interlayer region. This leads to the blistering of one‐monolayer thick domes, which stud the crystal surface and locally turn the dark bulk material into an efficient light emitter. The domes are stable (>2‐year lifetime) and robust, and host strong, complex strain fields. Lithographic techniques provide a means to engineer the formation process so that the domes can be produced with well‐ordered positions and sizes tunable from the nanometer to the micrometer scale, with important prospects for so far unattainable applications.