Van Der Waals Gap Research Articles

Bound exciton luminescence and phonon sideband analysis of iodine intercalated bulk 2H-MoSe2 crystals

Temperature dependence of the luminescence spectra of MoSe2 crystals intercalated with I2 molecules has been investigated in the temperature range 11–100 K. The spectrum of luminescence, which is caused by the recombination of excitons bound on iodine molecules embedded in the van der Waals gap (vdW), consists of zero-phonon doublet at an energy less by 0.1 eV than the width of the indirect band gap of the host crystal, and its phonon replicas. The distance between the spectral lines of this A-B doublet constitutes Δ AB =5.6 meV. From the temperature dependence of the ratio of the A and B lines intensities, it was found, that the rate of radiative recombination of the exciton state B, which is responsible for the short-wavelength line B (EB = 1.0416 eV), is 76 times higher than the recombination rate of the A state (EA = .0360 eV). Based on a comparative analysis of the structure of the luminescence spectra at different temperatures and the measured Raman spectra, it is shown that the observed nine peaks of the phonon sideband are formed by only two vibrational modes with frequencies νph1 = 144 cm−1 and νph2 = 190 cm−1. The 1st frequency corresponds to the vibrational mode due to the second-order Raman process, and the 2nd – to the local vibrational mode induced by the halogen molecule embedded in the layered crystal structure. Finally, fundamental possibilities provided by the intercalation of halogen molecules in the interface of the van der Waals heterojunctions to modify their electronic properties are considered.

Floquet engineering of magnetic topological insulator MnBi2Te4 films

Floquet engineering is an important way to manipulate the electronic states of condensed matter physics. Recently, the discovery of the magnetic topological insulator ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ and its family provided a valuable platform to study magnetic topological phenomena, such as the quantum anomalous Hall effect, the axion insulator state, and the topological magnetoelectric effect. In this work, based on the effective model and first-principles calculations in combination with the Floquet theory, we reveal that the circularly polarized light (CPL) induces the sign reversal of the Chern number of odd-septuple-layer (SL) ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ thin films. In contrast, the CPL drives the axion insulator state into the quantum anomalous Hall state in even-SL ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ thin films. More interestingly, if the topmost van der Waals gap between the surface layer and the below bulk in ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ films is slightly expanded, a high Chern number $|C|=2$ can be realized under the CPL. Our work demonstrates that the light can induce rich magnetic topological phases in ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ films, which might have potential applications in optoelectronic devices.

Narrowing the optical gap of CdPS3 single crystal via chemical intercalation using liquid ammonia method

The transition metal phosphorus trichalcogenides are fascinating two dimensional (2D) materials with expansive potential in optoelectronic devices, catalysis and energy storage. The van der Waals gap in structure gives a permit for rich intercalation chemistry, which could induce novel properties. In this work, we report the chemical intercalation in CdPS3 single crystals utilizing liquid ammonia method. It is demonstrated that the co-intercalation of alkali metal (Li or K) and NH3 molecule occurs during the intercalation reaction, leading to significantly enlarged layer distances (9.347–9.438 Å). The oxidation state of Cd in intercalated phases is clearly deceased compared to the parent CdPS3, which is ascribed to the charge transfer from the s orbitals of alkali metals to the 3d orbitals of Cd during intercalation. The charge transfer also largely narrows the optical gap of CdPS3 from 3.32 eV to 1.50 eV in K-intercalated phase, inducing visible-light response in this layered material. It is interesting to note that for the first time we report the successful chemical intercalation in transition metal phosphorus trichalcogenides with the single crystal form, instead of the polycrystalline sample. The intercalated products obtained by liquid ammonia method are still in single crystal form, which will facilitate the further mechanical or chemical exfoliation to synthesize new 2D materials.

Dredging the Charge-Carrier Transfer Pathway for Efficient Low-Dimensional Ruddlesden-Popper Perovskite Solar Cells.

Low-dimensional Ruddlesden-Popper (LDRP) perovskites still suffer from inferior carrier transport properties. Here, we demonstrate that efficient exciton dissociation and charge transfer can be achieved in LDRP perovskite by introducing γ-aminobutyric acid (GABA) as a spacer. The hydrogen bonding links adjacent spacing sheets in (GABA)2 MA3 Pb4 I13 (MA=CH3 NH3 + ), leading to the charges localized in the van der Waals gap, thereby constructing "charged-bridge" for charge transfer through the spacing region. Additionally, the polarized GABA weakens dielectric confinement, decreasing the (GABA)2 MA3 Pb4 I13 exciton binding energy as low as ≈73 meV. Benefiting from these merits, the resultant GABA-based solar cell yields a champion power conversion efficiency (PCE) of 18.73 % with enhanced carrier transport properties. Furthermore, the unencapsulated device maintains 92.8 % of its initial PCE under continuous illumination after 1000 h and only lost 3 % of its initial PCE under 65 °C for 500 h.

Dredging the Charge‐Carrier Transfer Pathway for Efficient Low‐Dimensional Ruddlesden‐Popper Perovskite Solar Cells

AbstractLow‐dimensional Ruddlesden‐Popper (LDRP) perovskites still suffer from inferior carrier transport properties. Here, we demonstrate that efficient exciton dissociation and charge transfer can be achieved in LDRP perovskite by introducing γ‐aminobutyric acid (GABA) as a spacer. The hydrogen bonding links adjacent spacing sheets in (GABA)2MA3Pb4I13 (MA=CH3NH3+), leading to the charges localized in the van der Waals gap, thereby constructing “charged‐bridge” for charge transfer through the spacing region. Additionally, the polarized GABA weakens dielectric confinement, decreasing the (GABA)2MA3Pb4I13 exciton binding energy as low as ≈73 meV. Benefiting from these merits, the resultant GABA‐based solar cell yields a champion power conversion efficiency (PCE) of 18.73 % with enhanced carrier transport properties. Furthermore, the unencapsulated device maintains 92.8 % of its initial PCE under continuous illumination after 1000 h and only lost 3 % of its initial PCE under 65 °C for 500 h.

2D/2D g-C3N4/ZnxCd1-xS Van der Waals heterojunctions modulation: Interfacial chemical bond accelerating charge separation for enhanced photocatalytic CO2 reduction

The fabrication of Van der Waals (VDW) heterojunctions between two-dimensional materials has recently sparked the curiosity of researchers due to their compact interfaces, low energy barriers and efficient interfacial electron mobility efficiency. However, the relatively weak VDW interactions and VDW gap inhibits the migration of carriers across layers. Herein, we developed a novel 2D/2D ZnxCd1-xS/g-C3N4 catalysts for regulating VDW heterojunctions by the interface engineering. ZnxCd1-xS nanosheets are strongly coordinated with g-C3N4, and the compact VDW heterojunctions and Zn/Cd molar ratios modulation induces Cd-N bonds to modulate the electronic structure at the interface. Evidenced by the experimental and theoretical calculation analysis, the constructed interfacial Cd-N bonds could function as a novel electron transport bridge between Zn0.8Cd0.2S and g-C3N4, enabling the photogenerated electron-hole pairs to be efficiently migrated directionally across the interlayer potential barrier. Therefore, Zn0.8Cd0.2S/g-C3N4 exhibits the enhanced CO2 reduction performance that is 3.2 times better than that of pure g-C3N4 without the usage of co-catalysts or sacrificial agents. This work introduces an avenue to regulate Van der Waals heterojunctions and provides an efficient photocatalyst for green energy conversion.

Improving the brightness of quasi-2D perovskite light-emitting diode by controlling stoichiometric ratio of perovskite emission layer

The quasi-two-dimensional (quasi-2D) perovskite possesses good film coverage and high photoluminescence quantum efficiency, and exhibits a characteristic of high-level radiation recombination. However, the charge transferring from quasi-2D perovskites is not comparable to that of three-dimensional perovskites due to the insulating nature of large organic cations, thus limits the brightness of quasi-2D perovskite light-emitting diodes (PLEDs). In this work, phenylethylammonium bromide (PEABr) and formamidine hydrobromide (FABr) were strategically introduced as two spacer cations to form quasi-2D perovskite phase, thus weakened the van der Waals gap between individual perovskite layers. The enhanced coupling strength between each layer promoted the energy transfer efficiency and led to an increasing radiative recombination rate of the perovskite films. Meanwhile, an optimized molar ratio of PEABr: FABr: PbBr2 improved perovskite film morphology and passivated its defects, which reduced the nonradiative recombination loss of quasi-2D PLEDs. As a result, a maximum brightness of 32,368 cd m − 2 can be obtained for this kind of devices.

Collective Magnetic Behavior in Vanadium Telluride Induced by Self-Intercalation

Self-intercalation of native magnetic atoms within the van der Waals (vdW) gap of layered two-dimensional (2D) materials provides a degree of freedom to manipulate magnetism in low-dimensional systems. Among various vdW magnets, the vanadium telluride is an interesting system to explore the interlayer order-disorder transition of magnetic impurities due to its flexibility in taking nonstoichiometric compositions. In this work, we combine high-resolution scanning transmission electron microscopy (STEM) analysis with density functional theory (DFT) calculations and magnetometry measurements, to unveil the local atomic structure and magnetic behavior of V-rich V1+xTe2 nanoplates with embedded V3Te4 nanoclusters grown by chemical vapor deposition (CVD). The segregation of V intercalations locally stabilizes the self-intercalated V3Te4 magnetic phase, which possesses a distorted 1T'-like monoclinic structure. This phase transition is controlled by the electron doping from the intercalant V ions. The magnetic hysteresis loops show that the nanoplates exhibit superparamagnetism, while the temperature-dependent magnetization curves evidence a collective superspin-glass magnetic behavior of the nanoclusters at low temperature. Using four-dimensional (4D) STEM diffraction imaging, we reveal the formation of collective diffuse magnetic domain structures within the sample under the high magnetic fields inside the electron microscope. Our results shed light on the studies of dilute magnetism at the 2D limit and on strategies for the manipulation of magnetism for spintronic applications.

Self-Intercalated 1T-FeSe2 as an Effective Kagome Lattice.

In kagome lattice, with the emergence of Dirac cones and flat band in electronic structure, it provides a versatile ground for exploring intriguing interplay among frustrated geometry, topology and correlation. However, such engaging interest is strongly limited by available kagome materials in nature. Here we report on a synthetic strategy of constructing kagome systems via self-intercalation of Fe atoms into the van der Waals gap of FeSe2 via molecular beam epitaxy. Using low-temperature scanning tunneling microscopy, we unveil a kagome-like morphology upon intercalating a 2 × 2 ordered Fe atoms, resulting in a stoichiometry of Fe5Se8. Both the bias-dependent STM imaging and theoretical modeling calculations suggest that the kagome pattern mainly originates from slight but important reconstruction of topmost Se atoms, incurred by the nonequivalent subsurface Fe sites due to the intercalation. Our study demonstrates an alternative approach of constructing artificial kagome structures, which envisions to be tuned for exploring correlated quantum states.

Ultralow contact resistance in organic transistors via orbital hybridization

Organic field-effect transistors (OFETs) are of interest in unconventional form of electronics. However, high-performance OFETs are currently contact-limited, which represent a major challenge toward operation in the gigahertz regime. Here, we realize ultralow total contact resistance (Rc) down to 14.0 Ω ∙ cm in C10-DNTT OFETs by using transferred platinum (Pt) as contact. We observe evidence of Pt-catalyzed dehydrogenation of side alkyl chains which effectively reduces the metal-semiconductor van der Waals gap and promotes orbital hybridization. We report the ultrahigh performance OFETs, including hole mobility of 18 cm2 V−1 s−1, saturation current of 28.8 μA/μm, subthreshold swing of 60 mV/dec, and intrinsic cutoff frequency of 0.36 GHz. We further develop resist-free transfer and patterning strategies to fabricate large-area OFET arrays, showing 100% yield and excellent variability in the transistor metrics. As alkyl chains widely exist in conjugated molecules and polymers, our strategy can potentially enhance the performance of a broad range of organic optoelectronic devices.

A Transient Pseudo-Capacitor Using a Bioderived Ionic Liquid with Na Ions.

A pseudo-capacitor with transient behavior is applied in implantable, disposable, and bioresorbable devices, incorporating an Na ion-doped bioderived ionic liquid, molybdenum trioxide (MoO3 )-covered molybdenum foil, and silk sheet as the electrolyte, electrode, and separator, respectively. Sodium lactate is dissolved in choline lactate as a source of Na ions. The Experimental results reveal that the Na ions are intercalated into the van der Waals gaps in MoO3 , and the pseudo-capacitor shows an areal capacitance (1.5 mFcm-2 ) that is three times larger than that without the Na ion. The fast ion diffusion of the electrolyte and the low resistance of the MoO3 and Mo interface result in an equivalent series resistance of 96 Ω. A cycle test indicates that the pseudo-capacitor exhibited a high capacitance retention of 82.8% after 10000 cycles. The transient behavior is confirmed by the dissolution of the pseudo-capacitor into phosphate-buffered saline solution after 101 days. Potential applications of transient pseudo-capacitors include electronics without the need for device retrieval after use, including smart agriculture, implantable, and wearable devices.

Molecular Bridging Strategy Enables High Performance and Stable Quasi-2D Perovskite Light-Emitting Devices

Quasi-two-dimensional (quasi-2D) Ruddlesden–Popper (RP) perovskites have attracted extraordinary attention due to their favorable energy funnel structures and outstanding optical performance. However, quasi-2D perovskites commonly contain the disordered mixture of phases, which results in low energy transfer between phases and subsequently limits the performance of the device. In this work, 4′-(Aminomethyl)-biphenyl-3-carboxylic acid (4-ABCA) was introduced into PEA2(FAPbBr3)2PbBr4 (PEA = phenethylammonium) film. The experimental and theoretical investigations demonstrate that 4-ABCA can coordinate Pb2+ and Br– with its carboxyl and amion groups, respectively, which reconstructs the phase distribution of PEA2(FAPbBr3)2PbBr4 film, and simultaneously reduces the van der Waals gap, resulting in the efficient energy transfer of carriers. Finally, the external quantum efficiency (EQE) of 23.15% and the maximal brightness of 133560 cd/m2 were achieved in the green perovskite light-emitting diodes (PeLEDs) based on quasi-2D film by adding 4-ABCA. Notably, it is the highest brightness for the pure green quasi-2D PeLEDs reported in the literature.

Approaching the quantum limit in two-dimensional semiconductor contacts.

The development of next-generation electronics requires scaling of channel material thickness down to the two-dimensional limit while maintaining ultralow contact resistance1,2. Transition-metal dichalcogenides can sustain transistor scaling to the end of roadmap, but despite a myriad of efforts, the device performance remains contact-limited3-12. In particular, the contact resistance has not surpassed that of covalently bonded metal-semiconductor junctions owing to the intrinsic van der Waals gap, and the best contact technologies are facing stability issues3,7. Here we push the electrical contact of monolayer molybdenum disulfide close to the quantum limit by hybridization of energy bands with semi-metallic antimony ([Formula: see text]) through strong van der Waals interactions. The contacts exhibit a low contact resistance of 42 ohm micrometres and excellent stability at 125 degrees Celsius. Owing to improved contacts, short-channel molybdenum disulfide transistors show current saturation under one-volt drain bias with an on-state current of 1.23 milliamperes per micrometre, an on/off ratio over 108 and an intrinsic delay of 74 femtoseconds. These performances outperformed equivalent silicon complementary metal-oxide-semiconductor technologies and satisfied the 2028 roadmap target. We further fabricate large-area device arrays and demonstrate lowvariability in contact resistance, threshold voltage, subthreshold swing, on/off ratio, on-state current and transconductance13. The excellent electrical performance, stability and variability make antimony ([Formula: see text]) a promising contact technology for transition-metal-dichalcogenide-based electronics beyond silicon.

One-dimensional semimetal contacts to two-dimensional semiconductors

Two-dimensional (2D) semiconductors are promising in channel length scaling of field-effect transistors (FETs) due to their excellent gate electrostatics. However, scaling of their contact length still remains a significant challenge because of the sharply raised contact resistance and the deteriorated metal conductivity at nanoscale. Here, we construct a 1D semimetal-2D semiconductor contact by employing single-walled carbon nanotube electrodes, which can push the contact length into the sub-2 nm region. Such 1D–2D heterostructures exhibit smaller van der Waals gaps than the 2D–2D ones, while the Schottky barrier height can be effectively tuned via gate potential to achieve Ohmic contact. We propose a longitudinal transmission line model for analyzing the potential and current distribution of devices in short contact limit, and use it to extract the 1D–2D contact resistivity which is as low as 10−6 Ω·cm2 for the ultra-short contacts. We further demonstrate that the semimetal nanotubes with gate-tunable work function could form good contacts to various 2D semiconductors including MoS2, WS2 and WSe2. The study on 1D semimetal contact provides a basis for further miniaturization of nanoelectronics in the future.

Two-Dimensional Iron Phosphorus Trisulfide as a High-Capacity Cathode for Lithium Primary Battery.

Metal phosphorus trichalcogenide (MPX3) materials have aroused substantial curiosity in the evolution of electrochemical storage devices due to their environment-friendliness and advantageous X-P synergic effects. The interesting intercalation properties generated due to the presence of wide van der Waals gaps along with high theoretical specific capacity pose MPX3 as a potential host electrode in lithium batteries. Herein, we synthesized two-dimensional iron thio-phosphate (FePS3) nanoflakes via a salt-template synthesis method, using low-temperature time synthesis conditions in single step. The electrochemical application of FePS3 has been explored through the construction of a high-capacity lithium primary battery (LPB) coin cell with FePS3 nanoflakes as the cathode. The galvanostatic discharge studies on the assembled LPB exhibit a high specific capacity of ~1791 mAh g-1 and high energy density of ~2500 Wh Kg-1 along with a power density of ~5226 W Kg-1, some of the highest reported values, indicating FePS3's potential in low-cost primary batteries. A mechanistic insight into the observed three-staged discharge mechanism of the FePS3-based primary cell resulting in the high capacity is provided, and the findings are supported via post-mortem analyses at the electrode scale, using both electrochemical- as well as photoelectron spectroscopy-based studies.

Cleavable crystals, crystal structure, and magnetic properties of the NbFe1+xTe3 layered van der Waals telluride.

Transition metal-based two-dimensional nanomaterials with competing magnetic states are at the cutting edge of spintronic and low-power memory devices. In this paper, we present a Fe-rich NbFe1+xTe3 layered telluride (x ≈ 0.5), which shows an interplay of spin-glass and antiferromagnetic states below the Néel temperature of 179 K. The compound has a layered crystal structure, where the NbFeTe3 layers are terminated by the Te atoms and van der Waals gaps. Bulk single crystals grown by chemical vapor transport reactions possess the (1̄01) cleavage plane suitable for the exfoliation of two-dimensional nanomaterials. Combination of high-resolution transmission electron microscopy and powder X-ray diffraction reveals the zigzag ladders of Fe atoms inside the structural layers, as well as complementary zigzag chains of the partially occupied Fe positions in the interstitial region. Fe atoms carry large effective magnetic moment of 4.85(3)μB per atom in the paramagnetic state yielding intriguing magnetic properties of NbFe1+xTe3. They include frozen spin-glass state at low temperatures and spin-flop transition in high magnetic fields indicating promising flexibility of the magnetic system and its potential control by magnetic field or gate tuning in the spintronic devices and heterostructures.

The rise of quasi-2D Dion-Jacobson perovskites for photovoltaics.

With the advance of nanotechnology, the past couple of years have witnessed the fast development of quasi two-dimensional (2D) halide perovskites, which exhibit outstanding long-term stability against moisture and heat, compared with their three-dimensional (3D) counterparts. As one of the most common structures in 2D halide perovskites, quasi-2D Dion-Jacobson (DJ) perovskites show multiple-quantum-well structures with n layers of [BX6]4- octahedral inorganic sheets sandwiched by two layers of diammonium spacers, thus exhibiting superior structural stability due to the elimination of van der Waals gaps. Thanks to the achievement of high power conversion efficiency accompanied by impressive stability, quasi-2D DJ perovskite solar cells (PSCs) have recently drawn extensive attention in the field. This review first introduces the fundamental understanding of quasi-2D DJ halide perovskites, including their superior stability, high exciton binding energy, and compositional flexibility and tunable properties. We then summarize detailed strategies to prepare high-quality quasi-2D DJ perovskites for PSCs, encompassing compositional engineering, solvent engineering, additive addition, and annealing processes. Moreover, the surface/interface modification and 2D-3D hybrid perovskite heterojunction are also discussed, for providing strategies to optimize the fabrication of quasi-2D DJ PSCs. Lastly, current challenges and perspectives toward the future development of quasi-2D DJ perovskites for photovoltaics are outlined.

Realizing avalanche criticality in neuromorphic networks on a 2D hBN platform.

Networks and systems which exhibit brain-like behavior can analyze information from intrinsically noisy and unstructured data with very low power consumption. Such characteristics arise due to the critical nature and complex interconnectivity of the brain and its neuronal network. We demonstrate a system comprising of multilayer hexagonal boron nitride (hBN) films contacted with silver (Ag), which can uniquely host two different self-assembled networks, which are self-organized at criticality (SOC). This system shows bipolar resistive switching between the high resistance state (HRS) and the low resistance state (LRS). In the HRS, Ag clusters (nodes) intercalate in the van der Waals gaps of hBN forming a network of tunnel junctions, whereas the LRS contains a network of Ag filaments. The temporal avalanche dynamics in both these states exhibit power-law scaling, long-range temporal correlation, and SOC. These networks can be tuned from one to another with voltage as a control parameter. For the first time, two different neural networks are realized in a single CMOS compatible, 2D material platform.

Two-dimensional layered Dion–Jacobson phase organic–inorganic tin iodide perovskite field-effect transistors

The single BDA2+layer in D–J BDASnI4eliminates the van der Waals gap in R–P perovskites, enhancing the out-of-plane charge transport and structural stability. Polymer-gated BDASnI4FETs show improved performance and environmental stability.

Preparation and Characterization of Francisite Solid Solutions Cu3Bi(Se1–xTexO3)2O2Br (x = 0–1): Possibility for Francisites as Starting Materials for Oxide van der Waals Ferromagnets

We synthesized the solid solutions Cu3Bi(Se1–xTexO3)2O2Br (x = 0, 0.25, 0.5, 0.75, 1) for the first time and characterized their structures by X-ray diffraction (XRD) measurements and their magnetic properties by electron spin resonance (ESR), magnetic susceptibility, magnetization, and specific heat measurements as well as by density functional theory (DFT) calculations. The Néel temperature TN and the critical field μ0HC needed for the metamagnetic transition increase with x, while the magnetization at a given magnetic field and the Curie–Weiss temperature Θ decrease with x. We show that the tendency for the interlayer antiferromagnetic (AFM) coupling in francisites is not explained by interlayer spin exchange but by the interlayer high-spin orbital interaction that occur across the van der Waals (vdW) gaps, hence indicating that francisites are vdW ferromagnets. This is surprising because, so far, well-established vdW ferromagnets are either layered tellurides or layered iodides. The trends in TN, μ0HC, and the magnetization of Cu3Bi(Se1–xTexO3)2O2Br as a function of x are well explained by the interlayer AFM interactions, and that in the Θ is explained by the intralayer spin exchanges. We proposed how one might modify francisites to make their interlayer interactions ferromagnetic (FM), hence leading to oxide vdW ferromagnets.