Jiuxiang Dai

This review provides an overview of the latest progress in low-temperature vapor-phase growth of high-quality 2D metal chalcogenides (2D MCs) through various vapor-phase techniques and consolidates the diverse applications of the 2D MCs in electronics, optoelectronics, flexible devices, and catalysis etc. The current challenges and future research directions of this research field are also discussed.

Abstract

2D metal chalcogenides (MCs) have garnered significant attention from both scientific and industrial communities due to their potential in developing next-generation functional devices. Vapor-phase deposition methods have proven highly effective in fabricating high-quality 2D MCs. Nevertheless, the conventionally high thermal budgets required for synthesizing 2D MCs pose limitations, particularly in the integration of multiple components and in specialized applications (such as flexible electronics). To overcome these challenges, it is desirable to reduce the thermal energy requirements, thus facilitating the growth of various 2D MCs at lower temperatures. Numerous endeavors have been undertaken to develop low-temperature vapor-phase growth techniques for 2D MCs, and this review aims to provide an overview of the latest advances in low-temperature vapor-phase growth of 2D MCs. Initially, the review highlights the latest progress in achieving high-quality 2D MCs through various low-temperature vapor-phase techniques, including chemical vapor deposition (CVD), metal-organic CVD, plasma-enhanced CVD, atomic layer deposition (ALD), etc. The strengths and current limitations of these methods are also evaluated. Subsequently, the review consolidates the diverse applications of 2D MCs grown at low temperatures, covering fields such as electronics, optoelectronics, flexible devices, and catalysis. Finally, current challenges and future research directions are briefly discussed, considering the most recent progress in the field.

Nature Communications, Published online: 11 December 2023; doi:10.1038/s41467-023-43965-x

An ideal soluble 2DP macromolecule is synthesized from a fully conjugated 1D polymer and can be fully characterized by ¹H-NMR, GPC, SEM, AFM. This 2DP macromolecule has very small micropores (6.0 Å) inside the macromolecule, a large area (30 × 68 nm), high molecular weight (Mn = 2.80 × 10⁵), a small thickness (4.4 Å), and the highest oxygen permselectivity.

Abstract

If ideal 2D polymer (2DP) macromolecules with small pores that are similar in size to gas molecules, large areas, small thickness, and excellent membrane-forming ability are synthesized, ultimate gas separation membranes would be obtained. However, as far it is known, such ideal well-characterized 2DP macromolecules are not isolated. In this study, an ideal 2DP macromolecule is synthesized by using the successive three reactions (Glaser coupling, SCAT reaction, and the introduction of octyl groups), in which the conjugated framework structure is maintained, from a fully conjugated 1D polymer. Because this exfoliated 2DP is soluble, the macromolecular structure can be fully characterized by ¹H-NMR, GPC, SEM, AFM, and its dense membrane with no defects can be fabricated by the solvent cast method. This soluble 2DP macromolecule has very small micropores (6.0 Å) inside the macromolecule, a large area (30 × 68 nm by SEM and AFM), high molecular weight (Mn = 2.80 × 10⁵ by GPC), and a small thickness (4.4 Å by AFM). This membrane shows the highest oxygen permselectivity exceeding Robeson's upper line because of the high molecular sieving effect of the controlled small micropores.

2D nanotags with unique shape and chemical fingerprint can be produced from layered materials by electrochemical exfoliation. Manufacturers can tag products with a single nanotag and record and store its unique properties. Consumers can record the shape of their products nanotag by optical microscopy and check product authenticity by matching its shape with a genuine record.

Abstract

This work demonstrates the use of 2D materials (2DMs) as identification tags by exploiting their unique shape. Electrochemical exfoliation enables the production of large quantities of optically accessible 2DMs with diverse morphology and large lateral sizes up to 20 µm. Image processing techniques are used to facilitate shape identification and matching within a dataset of 500 unique nanosheets. Rotational and translation invariant shape matching with no false positive matches between over 100 000 unique shape pairings is shown. The approach enables individual nanosheets to be deposited onto products, such as packaging of luxury goods, pharmaceuticals, banknotes, etc., as a unique seal of authenticity. Quick inspection of the nanoscale tag by optical microscopy allows the shape to be compared against the genuine dataset, enabling unique identification. The optical features of 2D materials, such as Raman and/or photoluminescence signals can be used as an additional chemical fingerprint, making the anticounterfeiting solution very robust.

A deep-UV ferroelectricity nonlinear optical crystal LiNH₄SO₄ is explored. It exhibits short deep-UV absorption edge, small refractive index dispersion, and moderate NLO response, which indicates that LAS is a promising NLO crystal for deep-UV quasi-pahse-matching. This work highlights the potential of sulphates and provides new opportunities for deep-UV NLO materials.

Abstract

Phase matching is indispensable for high efficiency of second-order nonlinear optical (NLO) crystals. Thousands of these crystals rely on birefringence phase matching, whereas only a few of them are quasi-phase matching without the restriction of birefringence. Herein, using oriented seeds inch-sized LiNH₄SO₄ (LAS) single crystals are successfully grown. LAS is NLO-active with a very short deep-UV absorption edge of 171 nm and high laser damage threshold up to 1.47 GW cm⁻² at 1064 nm, 10 ns, but its birefringence is merely 0.0078@532 nm, which is too small to realize birefringence phase matching in deep-UV region. Fortunately, P−E hysteresis loop, variable-temperature NLO tests, and ferroelectric domain observations confirm that LAS is ferroelectric. Furthermore, LAS exhibits small refractive index dispersion resulting in a relatively long periodic length of 1.4 µm and its single domain structure can be easily obtained by electrical poling. These attributes make LAS a scarce NLO candidate for deep-UV quasi-phase matching. This work highlights the longly overlooked potential of sulfates and provides new opportunities for deep-UV NLO materials.

Both experimentally and theoretically it is demonstrated that the magnetization of a 2D dilute magnetic semiconductor based on a V-doped transition metal dichalcogenide (TMD) monolayer or a 2D-TMD heterostructure composed of metallic (magnetic) and semiconducting (non-magnetic) TMD layers can be optically tuned. This establishes a novel direction for designing 2D-TMDs and heterostructures with optically tunable magnetic functionalities for next-generation nanodevices.

Abstract

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D-DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M-doped TX ₂, where M = V, Fe, and Cr; T = W, Mo; X = S, Se, and Te), may lead to innovative applications in spintronics, spin-caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D-TMD DMSs and heterostructures. By combining magneto-LC resonance (MLCR) experiments with density functional theory (DFT) calculations, we show that the magnetization can be enhanced using light in V-doped TMD monolayers (e.g., V-WS₂, V-WSe₂). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D-TMD heterostructures (VSe₂/WS₂, VSe₂/MoS₂), the significance of proximity, charge-transfer, and confinement effects in amplifying light-mediated magnetism is demonstrated. We attributed this to photon absorption at the TMD layer that generates electron–hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D-TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next-generation magneto-optic nanodevices.

Author(s): Z. Nie, L. Guery, E. B. Molinero, P. Juergens, T. J. van den Hooven, Y. Wang, A. Jimenez Galan, P. C. M. Planken, R. E. F. Silva, and P. M. Kraus

Photoinduced phase transitions in correlated materials promise diverse applications from ultrafast switches to optoelectronics. Resolving those transitions and possible metastable phases temporally are key enablers for these applications, but challenge existing experimental approaches. Extreme nonli…

[Phys. Rev. Lett. 131, 243201] Published Mon Dec 11, 2023

The 2D heterostructural MoN/MoC nanosheets with a precisely regulated interface prepared controllably from the bulk MoS₂ precursor show a large specific volumetric capacity (1045.3 F cm⁻³ at 1 A cm⁻³) and high-rate capability (702.8 F cm⁻³ at 10 A cm⁻³) due to fast charge transfer and enhanced ion absorption at the in-plane heterointerface.

Abstract

2D transition metal carbide/nitride heterostructures are emerging pseudocapacitive materials for supercapacitors (SCs); however, the lack of efficient synthesis methods and an in-depth understanding of the pseudocapacitive storage mechanism of these potentially important materials impede their applications in SCs. Herein, 2D MoN/MoC nanosheets with a precisely regulated interface are prepared controllably by a scalable salt-assisted method with bulk MoS₂ as the precursor. In operando infrared spectroscopy and electrochemical quartz crystal microbalance results reveal that the pseudocapacitance of the MoN/MoC nanosheets originates from the reversible reaction between Mo–N sites and H⁺ in the acidic electrolyte. Density-functional theory calculations and X-ray photoelectron spectroscopy disclose that the MoC/MoN heterointerface induces the internal electric field from the accumulated negative charges at the Mo–N sites by electron donation from MoC, leading to enhanced H⁺ adsorption at the Mo–N sites and superior pseudocapacitive storage. The heterostructured MoN/MoC nanosheets show a large volumetric capacity of 1045.3 F cm⁻³ at 1 A cm⁻³, high-rate capability of 702.8 F cm⁻³ at 10 A cm⁻³, and superior cyclability with capacity retention of 98% after 10,000 cycles, which outperform reported Mo-based carbides and nitrides. The results provide new insights into the development of high-performance 2D heterostructured materials for superior pseudocapacitive storage.

Oxidation behaviors of powder and bulk niobium silicide crystals are investigated using density functional theory (DFT), 3D kinetic Monte Carlo simulation, and experimental thermogravimetric analysis. Notably, the framework can be applied to other crystals, but there is no surety if it is suitable for all crystals or if it is feasible for some amorphous materials.

Abstract

To date, the oxidation behavior of crystal materials is not fully understood; additional research is needed to understand the oxidation of materials. Herein, density functional theory (DFT) calculations and a 3D kinetic Monte Carlo (KMC) model are used to investigate the infiltration and diffusion behaviors of oxygen atoms within the crystal. Oxygen molecules readily adsorbes on crystal surfaces of the material and rapidly dissociates, verified by both first-principles calculations and energy-dispersive spectrometer (EDS) results. The infiltration ability of oxygen atoms into the inner crystal layers is affected by the surrounding oxygen atom, lattice compactness, and other factors. Energy-barrier calculations show that crystal thin/dense layers have significant effects on the crystal oxidation process, so high-pressure technology is used to investigate this correlation experimentally. KMC calculations and thermogravimetric analyses (TGA) show the infiltration behavior of oxygen atoms in the main crystal plane (211) toward the inner layers has the highest proportion to the actual high-temperature oxidation behavior of the title material. The results of both the KMC calculations and thermal experiments show the material peeled off upon further oxidation, which accelerates oxidation. At the same time, high-pressure treatment increases the oxidation resistance of materials at lower temperatures (<600 °C).

This review presents novel strategies for strain-resistive designs and summarizes recent progress in stretchable electronics with strain-resistive performances. The strategies, including material design, structure engineering, and system integration, are summarized. Finally, challenges and perspectives regarding the development of strain-resistive stretchable electronics are discussed.

Abstract

Stretchable electronics have attracted tremendous attention amongst academic and industrial communities due to their prospective applications in personal healthcare, human-activity monitoring, artificial skins, wearable displays, human-machine interfaces, etc. Other than mechanical robustness, stable performances under complex strains in these devices that are not for strain sensing are equally important for practical applications. Here, a comprehensive summarization of recent advances in stretchable electronics with strain-resistive performance is presented. First, detailed overviews of intrinsically strain-resistive stretchable materials, including conductors, semiconductors, and insulators, are given. Then, systematic representations of advanced structures, including helical, serpentine, meshy, wrinkled, and kirigami-based structures, for strain-resistive performance are summarized. Next, stretchable arrays and circuits with strain-resistive performance, that integrate multiple functionalities and enable complex behaviors, are introduced. This review presents a detailed overview of recent progress in stretchable electronics with strain-resistive performances and provides a guideline for the future development of stretchable electronics.

The Cu_2-xSe-MoSe₂ edge-epitaxial heterostructure exhibits enhanced HER performance in comparison to the Cu_2-xSe@1T/2H-MoSe₂ core@shell heterostructure. This outcome substantiates the superiority of the edge-epitaxial configuration by exposing more active sites and improved electron transfer efficiency. Additionally, electron transfer from Cu_2-xSe to MoSe₂ results in an electron-rich state, further improving the HER performance.

Abstract

The exposure of active edge sites of transition metal dichalcogenide (TMD) in TMD-based heterostructures is essential to enhance the catalytic activity toward electrochemical catalytic hydrogen evolution (HER). The construction of TMD-based edge-epitaxial heterostructures can maximally expose the active edge sites. However, owing to the 2D crystal structures, it remains a great challenge to vertically align layered TMDs on non-layered metal chalcogenides. Herein, the synthesis of Cu_2-xSe-MoSe₂ edge-epitaxial heterostructure is reported by a facile one-pot wet-chemical method. A high density of MoSe₂ nanosheets grown vertically to the <111>_Cu2-xSe on the surface of Cu_2-xSe nanocrystals is observed. Such edge-epitaxial configuration allows the exposure of abundant active edge sites of MoSe₂ and enhances the changer transfer between MoSe₂ and Cu_2-xSe. As a result, the obtained Cu_2-xSe-MoSe₂ epitaxial heterostructures show excellent HER performance as compared to that of Cu_2-xSe@1T/2H-MoSe₂ core@shell heterostructure with similar size. This work not only offers a novel approach for designing efficient electrochemical catalysis but also enriches the diversity of TMD-based heterostructures, holding promise for various applications in the future.

A new non-thermal plasma method is used to fabricate a TiO₂/Ti₃O₄ hetero-phase bilayer in just seconds. The high-concentration electron gas at the heterointerface condenses into 2D electron liquid (2DEL) at room temperature. The 2DEL shields the scattering between electrons and phonons, allowing electrons to flow “adiabatically”. These features are suitable for low-loss electronics or plasmonics and hot electron utilization.

Abstract

Interfaces of metal oxide heterojunctions display a variety of intriguing physical properties that enable novel applications in spintronics, quantum information, neuromorphic computing, and high-temperature superconductivity. One such LaAlO₃/SrTiO₃ (LAO/STO) heterojunction hosts a 2D electron liquid (2DEL) presenting remarkable 2D superconductivity and magnetism. However, these remarkable properties emerge only at very low temperatures, while the heterostructure fabrication is challenging even at the laboratory scale, thus impeding practical applications. Here, a novel plasma-enabled fabrication concept is presented to develop the TiO₂/Ti₃O₄ hetero-phase bilayer with a 2DEL that exhibits features of a weakly localized Fermi liquid even at room temperature. The hetero-phase bilayer is fabricated by applying a rapid plasma-induced phase transition that transforms a specific portion of anatase TiO₂ thin film into vacancy-prone Ti₃O₄ in seconds. The underlying mechanism relies on the screening effect of the achieved high-density electron liquid that suppresses the electron-phonon interactions. The achieved “adiabatic” electron transport in the hetero-phase bilayer offers strong potential for low-loss electric or plasmonic circuits and hot electron harvesting and utilization. These findings open new horizons for fabricating diverse multifunctional metal oxide heterostructures as an innovative platform for emerging clean energy, integrated photonics, spintronics, and quantum information technologies.

Anhydrous rare-earth trichlorides RECl₃ with high purity and yield are obtained by reacting rare-earth oxides RE ₂O₃ with aluminum chloride AlCl₃ containing ionic liquids at 175 °C. The easy-to-perform process provides a reliable and scalable alternative to the production of these difficult-to-access anhydrous chlorides. In contrast to previous approaches, neither high temperatures nor highly toxic substances are required.

Abstract

Wide applications of anhydrous rare-earth (RE) trichlorides RECl₃ in organometallic chemistry, for the synthesis of optical and magnetic materials, and as catalysts require a facile approach for their synthesis. The known methods use or produce toxic substances, are complicated and have limited reliability and upscaling. It has been shown that task-specific ionic liquids (ILs) can dissolve many metal oxides without special reaction conditions at moderate temperature, making the metals accessible to downstream chemistry. Using imidazolium chloridoaluminate ILs, pure crystalline anhydrous RECl₃ (RE=La−Nd, Sm−Dy) can be synthesized in one step from RE oxides in high yield. The Lewis acidic IL acts as solvent and reaction partner. The by-product [Al₄O₂Cl₁₀]²⁻, which was detected spectroscopically, remains in solution. The reacted IL can be removed quantitatively by washing. ILs with various imidazolium cations and AlCl₃ content and the effect of temperature and reaction time were tested.

Nature Electronics, Published online: 08 December 2023; doi:10.1038/s41928-023-01094-9

Nature Electronics, Published online: 08 December 2023; doi:10.1038/s41928-023-01090-z

Novel topological semimetals, exemplified by NbIrTe₄, have opened new avenues in terahertz technology, driven by their intriguing non-equilibrium properties. This study highlights the selective growth of NbIrTe₄ and vdW stacking to form heterostructures, enabling self-powered terahertz photodetection at room temperature. The results reveal remarkable performance with high responsivity, low noise, and fast response, offering promising prospects in terahertz photon harvesting.

Abstract

The emergence of novel topological semimetal materials, accompanied by exotic non-equilibrium properties, not only provides a fertile playground for a fundamental level of interest but also opens exciting opportunities for inventing new applications by making use of different light-induced effects such as nonlinear optics, optoelectronics, especially for the highly pursued terahertz (THz) technology due to the gapless electronic structures. Exploring type-II Weyl semimetal endowed with the richness of quantum wavefunction and peculiar band structure, underlie strong nonlinear coupling with THz waves. Here, the selective growth of type-II Weyl semimetal NbIrTe₄ by means of a self-flux approach is reported, which hosts strongly tilted Weyl cones and exotic Fermi arcs. The oscillating THz field induced by the antenna is engineered in terms of planar metal-topological semimetal-metal structure, along with van der Waals stacking, which allows for self-powered photodetection at room temperature. The results elucidate the superior performance of NbIrTe₄-graphene heterostructure-based photodetectors with responsivity up to 264.6 V W⁻¹ at 0.30 THz, fast response of 1 µs as well as low noise equivalent power ˂0.28 nW Hz^−0.5 is achieved, already exhibiting high-quality imaging at THz frequency. The results promise superb impacts in exploring topological Weyl semimetals for efficient low-energy photon harvesting.

An efficient “top-down” method is used for processing natural wood into RTP films with a lifetime of ≈241.9 ms. Specifically, natural wood is partially delignified and then converted to W-film with a tensile strength of 273.6 MPa via mechanical pressing.

Abstract

Producing mechanic robust and sustainable room temperature phosphorescent (RTP) films in a convenient manner is a fundamental requirement but remains challenging. Here, an efficient “top-down” method for processing natural wood into RTP films is developed. Specifically, natural wood is partially delignified to increase its processability in the first step. After that, the treated wood is converted to W-film with a tensile strength of 273.6 MPa via mechanical pressing. The lignin units are well confined by cellulose in the W-film, triggering RTP emission with a lifetime of ≈241.9 ms. Additionally, the W-film exhibits good processability and energy transfer properties with Rhodamine B (RhB). Therefore, a series of multifunctional afterglow 2D and 3D RTP materials are constructed using W-film as a building block. This research is expected to result in a convenient and sustainable method for the preparation of RTP films.

The dual-floating van der Waals photodiode fabricated with MoTe₂ and MoS₂ 2D semiconductor exhibits robust linear photoresponse under photovoltaic mode from visible (405 nm) to near-infrared (1600 nm) band. The performances of linear dynamic range ≈100 dB, responsivity ≈1.57 A W⁻¹, detectivity ≈4.28 × 10¹¹ Jones, and response speed ≈30 µs are achieved.

Abstract

Two-dimensional (2D) material photodetectors have gained great attention as potential elements for optoelectronic applications. However, the linearity of the photoresponse is often compromised by the carrier interaction, even in 2D photodiodes. In this study, a new device concept of dual-floating van der Waals heterostructures (vdWHs) photodiode is presented by employing ambipolar MoTe₂ and n-type MoS₂ 2D semiconductors. The presence of type II heterojunctions on both sides of channel layers effectively depletes carriers and restricts the photocarrier trapping within the channel layers. As a result, the device exhibits robust linear photoresponse under photovoltaic mode from the visible (405 nm) to near-infrared (1600 nm) band. With the built-in electric field of the vdWHs, a linear dynamic range of ≈100 dB, responsivity of ≈1.57 A W⁻¹, detectivity of ≈4.28 × 10¹¹ Jones, and response speed of ≈30 µs are achieved. The results showcase a promising device concept with excellent linearity toward fast and low-loss detection, high-resolution imaging, and logic optoelectronics.

A nanographene with a tri-N-doped cavity was synthesized by photo-induced cyclization. In comparison with nitrogen-doping at the edge, this tri-N-doped holey nanographene exhibited markedly reduced basicity and selective affinity toward Ag⁺. This nanographene with a N-doped cavity provides a precise model for understanding the binding in the nano-confined defects of graphenic materials.

Abstract

Nitrogen-doped cavities are pervasive in graphenic materials, and represent key sites for catalytic and electrochemical activity. However, their structures are generally heterogeneous. In this study, we present the synthesis of a well-defined molecular cutout of graphene featuring N-doped cavity. The graphitization of a macrocyclic pyridinic precursor was achieved through photochemical cyclodehydrochlorination. In comparison to its counterpart with pyridinic nitrogen at the edges, the pyridinic nitrogen atoms in this nanographene cavity exhibit significantly reduced basicity and selective binding to Ag⁺ ion. Analysis of the protonation and coordination equilibria revealed that the tri-N-doped cavity binds three protons, but only one Ag⁺ ion. These distinct protonation and coordination behaviors clearly illustrate the space confinement effect imparted by the cavities.

The ultrathin van der Waals (vdW) LaOCl is synthesized by controlling the growth kinetics. Due to the considerable dielectric properties of LaOCl and its dangling-bond-free surface, the MoS₂ field-effect transistor (FET) with vdW LaOCl dielectric exhibits ultralow hysteresis. LaOCl possesses the tremendous potential to act as an ideal gate dielectric for two-dimensional FETs.

Abstract

Downsizing silicon-based transistors can result in lower power consumption, faster speeds, and greater computational capacity, although it is accompanied by the appearance of short-channel effects. The integration of high-mobility 2D semiconductor channels with ultrathin high dielectric constant (high-κ) dielectric in transistors is expected to suppress the effect. Nevertheless, the absence of a high-κ dielectric layer featuring an atomically smooth surface devoid of dangling bonds poses a significant obstacle in the advancement of 2D electronics. Here, ultrathin van der Waals (vdW) lanthanum oxychloride (LaOCl) dielectrics are successfully synthesized by precisely controlling the growth kinetics. These dielectrics demonstrate an impressive high-κ value of 10.8 and exhibit a remarkable breakdown field strength (E _bd) exceeding 10 MV cm⁻¹. Remarkably, the conventional molybdenum disulfide (MoS₂) field-effect transistor (FET) featuring a dielectric made of LaOCl showcases an almost negligible hysteresis when compared to FETs employing alternative gate dielectrics. This can be attributed to the flawlessly formed vdW interface and excellent compatibility established between LaOCl and MoS₂. These findings will motivate the further exploration of rare-earth oxychlorides and the development of more-than-Moore nanoelectronic devices.

Abstract

Stacking single layers of atoms on top of each other provides a fundamental way to achieve novel material systems and engineer their physical properties, which offers opportunities for exploring fundamental physics and realizing next-generation optoelectronic devices. Among the two-dimensional (2D)-stacked systems, transition metal dichalcogenide (TMDC) heterostructures are particularly attractive because they host tightly-bonded interlayer excitons which possess various novel and appealing properties. These interlayer excitons have drawn significant research attention and hold high potential for the application in unique optoelectronic devices, such as polarization- and wavelength-tunable single photon emitters, valley Hall transistors, and possible high-temperature superconductors. The development of these devices requires a comprehensive understanding of the fundamental properties of these interlayer excitons and the impact of electric fields on their behaviors. In this review, we summarize the recent advances on the understanding of interlayer exciton dynamics under electric fields in TMDC heterostructures. We put emphasis on the electrical modulation of interlayer excitons’ emission, the valley Hall transport of charge carriers after the separation of interlayer excitons by an electric field, and the correlation physics of interlayer excitons and charges under electrical doping and tuning. Challenges and perspectives are finally discussed for the application of TMDC heterostructures in future optoelectronics.

Nature Materials, Published online: 07 December 2023; doi:10.1038/s41563-023-01746-3