One-dimensional Van Der Waals Research Articles

One-Dimensional van der Waals Heterojunction Comprising Carbon Nanotube Half-Wrapped in Boron Nitride Nanotube: Deep Investigation of Thermal Rectification.

One-dimensional van der Waals (vdWs) heterostructures are celebrated for their exceptional thermal management capabilities, garnering significant research interest. Consequently, our research focused on the one-dimensional vdWs heterojunction comprising carbon nanotube half-wrapped in boron nitride nanotube (BNCNT), specifically their thermal rectification (TR) properties. We employed non-equilibrium molecular dynamics to explore the TR mechanism and assess the impacts of temperature, strain, and coupling strength on heat flux and TR ratio. Our findings reveal that the backward heat flux demonstrates greater atomic vibration instability, as indicated by mean square displacement (MSD), compared to forward heat flux. This instability leads to a higher concentration of localized phonons, thereby diminishing the backward heat flux and enhancing TR. Additionally, we utilized MSD to shed light on the negative differential thermal resistance phenomenon and the influence of stress on forward and backward heat fluxes. Remarkably, TR ratios reached 344% at 3% strain and 400% at -1% strain. Calculations of phonon density of states revealed a competitive mechanism between in-plane and out-of-plane phonons coupling in the inner carbon nanotube and an overlap degree of out-of-plane phonon spectra between the inner carbon nanotube and outer boron nitride nanotube. This accounts for the differing trends in forward and backward heat fluxes as coupling strength χ increases, with TR ratios exceeding 1000% at χ = 7.5. This study provides vital insights for advancing one-dimensional vdWs thermal rectifiers.

Facet-governed frictional behavior in graphene/h-BN heteronanotubes

Superlubricity, a state characterized by ultra-low friction and nearly-zero wear, has conventionally been linked to planar incommensurate interfaces between heterogeneous van der Waals layered materials. In this work, our investigation introduces a novel perspective by highlighting the pivotal role of curvature in heterostructures. Employing our recently developed anisotropic interlayer force field, we demonstrate the profound impact of size and chirality on the morphological characteristics, specifically circumferential faceting, of heterogeneous graphene/h-BN nanotubes. This exploration unravels a consequential transition from a superlubric to a high-friction state, coinciding with the transformation of nanotube cross-sections from cylindrical to faceted shapes. The induction of a faceted scenario triggers redistribution of moiré-dependent distortion deformation during sliding, resulting in a transition from a stick-slip mode to a high-friction state. This stands in stark contrast to the smooth-sliding mode (superlubric state) observed in non-faceted heterogeneous nanotubes. Our discoveries provide valuable insights for the intentional design of quasi one-dimensional van der Waals metamaterials and devices with tunable tribological properties, and expand the current understanding of superlubricity.

One-dimensional van der Waals V2Se9 as a conversion-type anode material for advanced Li rechargeable batteries

Volume expansion of active materials during lithium storage is one of the main issues in Li-ion batteries (LIBs) that should be addressed. In particular, large volume changes and particle cracking are problematic for electrode materials based on conversion reaction, which are considered attractive anode materials with high energy densities. In this study, a one-dimensional van der Waals (1D vdW) material, V2Se9, is proposed as a conversion-type anode material for LIBs. The V2Se9 electrode material has a 1D chain molecular structure with vdW interactions between chains to form a bulk crystal structure. The spaces between the 1D chains act as structural buffers and provide short Li+ diffusion paths that can improve the electrochemical performance of the electrode materials. The V2Se9 electrode exhibits a high reversible capacity of 563.3 mAh g−1 at 100 mA g−1 and a high rate capability of 109.1 mAh g−1 at 3200 mA g−1. The capacity retention of V2Se9 electrodes is 83.72% after 100 cycles. Synchrotron X-ray diffraction and X-ray absorption spectroscopy reveal the formation of Li2Se and V metal, indicating that Li ion storage occurs via the conversion reaction. The structural stability of V2Se9 is demonstrated using SEM measurements of the discharged and charged V2Se9 electrodes during cycling. These results demonstrate the reaction mechanism of V2Se9 and highlight the potential of 1D vdW materials as high-performance anode materials in LIBs.

Giant intrinsic photovoltaic effect in one-dimensional van der Waals grain boundaries

The photovoltaic effect lies at the heart of eco-friendly energy harvesting. However, the conversion efficiency of traditional photovoltaic effect utilizing the built-in electric effect in p-n junctions is restricted by the Shockley-Queisser limit. Alternatively, intrinsic/bulk photovoltaic effect (IPVE/BPVE), a second-order nonlinear optoelectronic effect arising from the broken inversion symmetry of crystalline structure, can overcome this theoretical limit. Here, we uncover giant and robust IPVE in one-dimensional (1D) van der Waals (vdW) grain boundaries (GBs) in a layered semiconductor, ReS2. The IPVE-induced photocurrent densities in vdW GBs are among the highest reported values compared with all kinds of material platforms. Furthermore, the IPVE-induced photocurrent is gate-tunable with a polarization-independent component along the GBs, which is preferred for energy harvesting. The observed IPVE in vdW GBs demonstrates a promising mechanism for emerging optoelectronics applications.

Interlayer Charge Transfer Induced Electrical Behavior Transition in 1D AgI@sSWCNT van der Waals Heterostructures.

The emergence of one-dimensional van der Waals heterostructures (1D vdWHs) opens up potential fields with unique properties, but precise synthesis remains a challenge. The utilization of mixed conductive types of carbon nanotubes as templates has imposed restrictions on the investigation of the electrical behavior and interlayer interaction of 1D vdWHs. In this study, we efficiently encapsulated silver iodide in high-purity semiconducting single-walled carbon nanotubes (sSWCNTs), forming 1D AgI@sSWCNT vdWHs. We characterized the semiconductor-metal transition and increased the carrier concentration of individual AgI@sSWCNTs via sensitive dielectric force microscopy and confirmed the results through electrical device tests. The electrical behavior transition was attributed to an interlayer charge transfer, as demonstrated by Kelvin probe force microscopy. Furthermore, we showed that this method of synthesizing 1D heterostructures can be extended to other metal halides. This work opens the door for the further exploration of the electrical properties of 1D vdWHs.

One-dimensional van der Waals BiSBr: an anisotropic thermoelectric mineral.

The fascinating characteristics of one-dimensional van der Waals crystals (V-VI-VII) enable their wide functionality. In particular, their anisotropic carrier transport and low thermal conductivity are advantageous from a thermoelectric viewpoint. In a quest for the "electron crystal phonon glass" paradigm, the present work investigated the thermoelectric performance of BiSBr. Deep insights were gained into the structural and electronic properties that revealed the synergetic effect of the bonding heterogeneity, lone pair of electrons, and degenerated bands that accelerated favorable transportation. Consequently, a low lattice thermal conductivity (0.225 W m-1 K-1) and considerable power factor (3.471 mW m-1 K-2) and thus a high zT of 2.34 were noted in the x-direction (perpendicular to the chain) at 500 K in the case of the material with 1.5 × 1020 holes per cm3. These observations suggest that BiSBr is a plausible near-room-temperature anisotropic thermoelectric mineral.

One-dimensional van der Waals transition metal chalcogenide as an anode material for advanced lithium-ion batteries

1D vdW Nb2Se9 is a promising advanced anode material for LIBs due to superior electrochemical performance originating from its unique structural properties, which allow swift transport of Li ions and buffer the volume changes during Li-ion storage.

Coaxial boron nitride nanotubes as interfacial dielectric layers to lower interface trap density in carbon nanotube transistors

A semiconductor/dielectric interface is one of the dominant factors in device characteristics, and a variety of oxides with high dielectric constants and low interface trap densities have been used in carbon nanotube transistors. Given the crystal structure of nanotubes with no dangling bonds, there remains room to investigate unconventional dielectric materials. Here, we fabricate carbon nanotube transistors with boron nitride nanotubes as interfacial layers between channels and gate dielectrics, where a single semiconducting nanotube is used to focus on switching behaviors at the subthreshold regime. The subthreshold swing of 68 mV·dec−1 is obtained despite a 100-nm-thick SiO2 dielectric, corresponding to the effective interface trap density of 5.2 × 1011 cm−2·eV−1, one order of magnitude lower than those of carbon nanotube devices without boron nitride passivation. The interfacial layers also result in the mild suppression of threshold voltage variation and hysteresis. We achieve Ohmic contacts through the selective etching of boron nitride nanotubes with XeF2 gas, overcoming the trade-off imposed by wrapping the inner nanotubes. Negligible impacts of fluorinating carbon nanotubes on device performances are also confirmed as long as the etching is applied exclusively at source/drain regions. Our results represent an important step toward nanoelectronics that exploit the advantage of one-dimensional van der Waals heterostructures.

(Invited) Efficient Inner-to-Outer Wall Energy Transfer in Highly Pure Double-Wall Carbon Nanotubes Revealed by Detailed Spectroscopy

The coaxial stacking of two single-wall carbon nanotubes (SWCNTs) into a double-wall carbon nanotube (DWCNT), forming a so-called one-dimensional van der Waals structure, leads to synergetic effects that dramatically affect the optical and electronic properties of both layers [1]. In this work, we explore these effects in purified DWCNT samples by combining absorption, wavelength-dependent infrared fluorescence−excitation (PLE), and wavelength-dependent resonant Raman scattering (RRS) spectroscopy. Purified DWCNTs are obtained by careful solubilization that strictly avoids ultrasonication [2] or by electronic-type sorting [3], both followed by a density gradient ultracentrifugation to remove unwanted SWCNTs that could obscure the DWCNT characterization [2]. Chirality-dependent shifts of the radial breathing mode vibrational frequencies and transition energies of the inner and outer DWCNT walls with respect to their SWCNT analogues are determined by advanced two-dimensional fitting of RRS and PLE data of DWCNT and their reference SWCNT samples. This exhaustive data set verifies that fluorescence from the inner DWCNT walls of well-purified samples is severely quenched, as previously evidenced in single-tube experiments [4]. Moreover, we find that this quenching occurs through efficient energy transfer from the inner to the outer DWCNT walls [5]. Combined analysis of the PLE and RRS results further reveals that this transfer is dependent on the inner and outer wall chirality, and we identify the specific combinations dominant in our DWCNT samples. These obtained results demonstrate the necessity and value of a combined structural characterization approach including PLE and RRS spectroscopy for bulk DWCNT samples.[1] S. Cambré et al, Small 2021, 17, 2102585[2] M. Erkens et al, Carbon 2021, 185, 113[3] H. Li et al, Nature Nanotechnol. 2017, 12, 1176[4] D. Levshov et al, Phys. Rev. B 2017, 96, 195410[5] M. Erkens et al, ACS Nano 2022, 16, 16038

Horizontal Arrays of One-Dimensional van der Waals Heterostructures as Transistor Channels

The nanotube/dielectric interface plays an essential role in achieving superb switching characteristics of carbon nanotube-based transistors for energy-efficient computation. Formation of van der Waals heterostructures with hexagonal boron nitride nanotubes could be an effective means to reduce interface state density, but the need for isolating nanotubes during the formation of coaxial outer layers has hindered the fabrication of their horizontal arrays. Here, we develop a strategy to create isolated heterostructure arrays using aligned carbon nanotubes grown on a quartz substrate as starting materials. Air-suspended arrays of carbon nanotubes are prepared by a dry transfer technique and then used as templates for the coaxial wrapping of boron nitride nanotubes. We then fabricate the transistors, where boron nitride serves as interfacial layers between carbon nanotube channels and conventional gate dielectrics, showing hysteresis-free characteristics owing to the improved interfaces. We have also gained a deeper understanding of the strain applied on inner carbon nanotubes, as well as the inhomogeneity of the outer coating, by characterizing individual heterostructures over trenches and on a substrate surface. The device fabrication and characterization presented here essentially do not require elaborate electron microscopy, thus paving the way for the practical use of one-dimensional van der Waals heterostructures for nanoelectronics.

Direct Active Site at the Van der Waals Heterostructure Interface with Synthetic Drug Analogue N-Methylphenethylimine Ultrasensitivity.

CNT/organic probe-based chemiresistive sensors suffer from the problem of low sensitivity and poor stability due to the unstable and unfavorable CNT/organic probe interface. A new designing strategy of a one-dimensional van der Waals heterostructure was developed for ultrasensitive vapor sensing. By modifying the perylene diimide molecule at the bay region with phenoxyl and further Boc-NH- phenoxy side chains, a highly stable 1D VDW heterostructure SWCNT-probe molecule system was formed with ultrasensitivity and specificity. Interfacial recognition sites consisting of SWCNT and the probe molecule are responsible for the synergistical and excellent sensing response to MPEA molecules, which was proved by Raman, XPS, and FTIR characterizations together with dynamic simulation. Based on such a sensitive and stable VDW heterostructure system, the measured detection limit reached as low as 3.6 ppt for the synthetic drug analogue N-methylphenethylimine (MPEA) in the vapor phase, and the sensor showed almost no performance degradation even after 10 days. Furthermore, a miniaturized detector was developed for real-time monitoring of drug vapor detection.

Nanowires exfoliated from one-dimensional van der Waals transition metal trihalides and quadrihalides.

The exfoliation of van der Waals (vdW) materials has been widely used to fabricate two-dimensional (2D) materials. However, the exfoliation of vdW materials to isolate atomically thin nanowires (NWs) is an emerging research topic. In this letter, we identify a large class of transition metal trihalides (TMX3), which have one-dimensional (1D) vdW structures, i.e., they comprise columns of face-sharing TMX6 octahedral chains, whereas the chains are bound by weak vdW forces. Our calculations show that the single-chain and multiple-chain NWs constructed from these 1D vdW structures are stable. The calculated binding energies of the NWs are relatively small, suggesting that it is possible to exfoliate NWs from the 1D vdW materials. We further identify several 1D vdW transition metal quadrihalides (TMX4) that are candidates for exfoliation. This work opens a paradigm for exfoliating NWs from 1D vdW materials.

Strain-Tunable Electronic and Transport Properties of One-Dimensional Fibrous Phosphorus Nanotubes.

The one-dimensional van der Waals (1D vdW) material fibrous red phosphorus (FRP) nanotubes are a promising direct-bandgap semiconductor with high carrier mobility and anisotropic optical responses because of low deformation potential and dangling-bond-free anisotropic interface. Employing first-principles calculations, we captured the potential of 1D FRP nanotubes. The thermal stability of 1D FRP nanotubes was confirmed by phonon calculation. Meanwhile, Raman spectroscopy indicated the strong vibration mode (366 cm-1) is along the phosphorus nanotube. Interestingly, spatial anisotropy bandgaps were found along with various stacking orientations. The charge transport calculations showed that the 1D FRP nanotube has a high hole mobility (499.2 cm2 V-1 s-1), considering the weak acoustic phonon scattering. More importantly, we found that the hole mobility changes dramatically (down to 7.1 cm2 V-1 s-1) under the strain, and the strain-dependent charge transport property of 1D FRP nanotubes could be considered to have many potential applications for electronics, optoelectronics, and switching devices.

1D van der Waals heterostructures based on single-walled carbon nanotubes encapsulating linear carbon chains: Stability and charge transfer evidence from Raman spectroscopy

The emerging one-dimensional van der Waals heterostructures (1D vdWHTs) are raising a lot of enthusiasm due to the possibilities for creating novel physical properties. In this paper, we focus on single-walled carbon nanotubes (SWCNTs) encapsulating a series of hydrogen-terminated linear carbon chains (LCCs) of lengths ranging from 4 up to 40 carbon atoms as a template for 1D vdWHTs (LCCs@SWCNTs). We explored the structural properties and stability of the LCCs@SWCNTs systems. The optimal SWCNT diameter for which the resulting 1D vdWHTs are stable is determined using Lennard-Jones (LJ) potential. SWCNTs with diameters of around 0.75 nm are found most convenient to accommodate LCCs. Combining the molecular dynamics (MD), the density functional theory (DFT), bond polarizability model (BPM), and the spectral moment’s method (SMM), we computed the Raman spectra of the free LCCs and LCCs@(6,5) 1D vdWHTs. The stability and charge transfer in the LCCs@(6,5) systems are evinced by analyzing the Raman active modes of the LCCs, as well as the radial breathing mode (RBM) and G-band of the (6,5) SWCNT before and after filling. These findings provide benchmark theoretical data to understand both the Raman data of the LCCs and the confinement effect on those of the SWCNTs encasing LCCs which have not been clearly addressed before.

Van der Waals SWCNT@BN Heterostructures Synthesized from Solution-Processed Chirality-Pure Single-Wall Carbon Nanotubes.

Single-wall carbon nanotubes in boron nitride (SWCNT@BN) are one-dimensional van der Waals heterostructures that exhibit intriguing physical and chemical properties. As with their carbon nanotube counterparts, these heterostructures can form from different combinations of chiralities, providing rich structures but also posing a significant synthetic challenge to controlling their structure. Enabled by advances in nanotube chirality sorting, clean removal of the surfactant used for solution processing, and a simple method to fabricate free-standing submonolayer films of chirality pure SWCNTs as templates for the BN growth, we show it is possible to directly grow BN on chirality enriched SWCNTs from solution processing to form van der Waals heterostructures. We further report factors affecting the heterostructure formation, including an accelerated growth rate in the presence of H2, and significantly improved crystallization of the grown BN, with the BN thickness controlled down to one single BN layer, through the presence of a Cu foil in the reactor. Transmission electron microscopy and electron energy-loss spectroscopic mapping confirm the synthesis of SWCNT@BN from the solution purified nanotubes. The photoluminescence peaks of both (7,5)- and (8,4)-SWCNT@BN heterostructures are found to redshift (by ∼10 nm) relative to the bare SWCNTs. Raman scattering suggests that the grown BN shells pose a confinement effect on the SWCNT core.

Formation of One-Dimensional van der Waals Heterostructures via Self-Assembly of Blue Phosphorene Nanoribbons to Carbon Nanotubes

Van der Waals heterostructures composed of low-dimensional atomic layers host rich physics for new device applications, such as magic-angle twisted bilayer graphene and coaxial multi-walled hetero-nanotubes. Aside from exploring their abnormal physical behavior, fabrication of such structures also presents a great challenge to this area, owing to the subtle and sensitive interactions among neighboring layers. Here we show by molecular dynamics simulations that narrow blue phosphorene nanoribbons can be encapsulated into carbon nanotubes driven by van der Waals interactions and form one-dimensional heterostructures. It shows that by varying carbon nanotube diameters and nanoribbon width, the nanoribbons can either retain their original straight structures or twist into tubular structures. Wrapping phases are also observed for large-sized blue phosphorus. It is found that the underlying mechanism originates from the competition between van der Waals energy and bending energy induced by tube curvature. A phase diagram of the resultant 1D structure is thus obtained based on a simple analysis of energetics. The results are expected to stimulate further experimental efforts in fabricating one-dimensional van der Waals heterostructues with desired functionality.

Buckling behavior of ternary one-dimensional van der Waals heterostructures

One-dimensional van der Waals heterostructures (1D vdWHs) may suffer from external compression when applied in field-effect, light-emitting and photovoltaic devices. Ternary 1D vdWHs were recently reported to be successfully synthesized (Xiang et al 2020 Science 367, 537). In present work, the buckling behavior of ternary 1D vdWH consisting of an inner carbon nanotube, a middle boron nitride nanotube and an outer molybdenum disulfide nanotube is extensively investigated by using molecular dynamics simulations. We find that the composite can effectively enhance the capability of axial compression of the inner nanotubes. The 1D vdWH gradually loses its stability under uniaxial compression and the critical stress of buckling decreases as the temperature increases. Slenderness ratio α of 4.8 ≤ α ≤ 7.2 has a slight influence on the strength and stability of ternary 1D vdWH under axial compression. To obtain a 1D vdWH with best compressive stability and strength, there is an optimal diameter existing for any specific length. Our work provides guidance for the design of 1D vdWH with desired compressive stability.

Efficient Inner-to-Outer Wall Energy Transfer in Highly Pure Double-Wall Carbon Nanotubes Revealed by Detailed Spectroscopy.

The coaxial stacking of two single-wall carbon nanotubes (SWCNTs) into a double-wall carbon nanotube (DWCNT), forming a so-called one-dimensional van der Waals structure, leads to synergetic effects that dramatically affect the optical and electronic properties of both layers. In this work, we explore these effects in purified DWCNT samples by combining absorption, wavelength-dependent infrared fluorescence-excitation (PLE), and wavelength-dependent resonant Raman scattering (RRS) spectroscopy. Purified DWCNTs are obtained by careful solubilization that strictly avoids ultrasonication or by electronic-type sorting, both followed by a density gradient ultracentrifugation to remove unwanted SWCNTs that could obscure the DWCNT characterization. Chirality-dependent shifts of the radial breathing mode vibrational frequencies and transition energies of the inner and outer DWCNT walls with respect to their SWCNT analogues are determined by advanced two-dimensional fitting of RRS and PLE data of DWCNT and their reference SWCNT samples. This exhaustive data set verifies that fluorescence from the inner DWCNT walls of well-purified samples is severely quenched through efficient energy transfer from the inner to the outer DWCNT walls. Combined analysis of the PLE and RRS results further reveals that this transfer is dependent on the inner and outer wall chirality, and we identify the specific combinations dominant in our DWCNT samples. These obtained results demonstrate the necessity and value of a combined structural characterization approach including PLE and RRS spectroscopy for bulk DWCNT samples.

One-dimensional van der Waals material InSeI with large band-gap for optoelectronic applications

We experimentally demonstrate for the first time that the bulk InSeI crystals can be effectively delaminated by micromechanical exfoliation into a few nanometer-scale InSeI nanowires, due to the presence of weak van der Waals (vdWs) interaction within the crystal lattice. The exfoliated InSeI nanowires show one-dimensional (1D) bundle structure, and the work function of InSeI measured by Scanning Kelvin probe microscopy shows strong dependency on its thickness. The InSeI has a large bandgap corresponding to 2.12 eV, and when applied to a UV-detecting device, it was confirmed that the device with excellent sensitivity can be obtained, because there is little interference in the infrared and visible light regions. Based on these results, InSeI could be a excellent candidate as an additional 1D vdWs material for optoelectronics, including photodetectors or photovoltaic devices.

Fabricating one-dimensional van der Waals heterostructures on chirality-sorted single-walled carbon nanotubes

We previously demonstrated the experimental synthesis of one-dimensional van der Waals (1D vdW) heterostructures, where single-crystal boron nitride nanotubes (BNNT) and/or molybdenum disulfide nanotubes seamlessly wrap around a single-walled carbon nanotube (SWCNT) and form a coaxial hetero-nanotube with a diameter of only several nm. However, in that work, the inner-most templates are SWCNTs with mixed diameters and band structures, so the electronic property of the heterostructure is not controllable. In this work, we demonstrate the possibility of building 1D vdW heterostructures on atomically precise, single-chirality SWCNTs. This is achieved by a long route of dispersing, sorting, re-depositing and cleaning SWCNTs before performing the synthesis of 1D heterostructures. We employ five different single-chirality SWCNTs and confirm that 1D heterostructures can be fabricated on these solution-sorted SWCNT templates if the surfactant can be readily removed. The obtained heterostructures show a very minor variation in inner diameters, suggesting the high uniformity of the starting SWCNT template. Solution processes are found to have minor side effects and the defects are largely healed after the high temperature BNNT growth. As a result, the obtained heterostructures show a high crystal quality according to the spectroscopic and microscopic characterizations. Finally, we perform a structural analysis using (6, 5) SWCNT based 1D heterostructures as the example. Electron diffraction reveals an abundance of 80% of (6, 5) SWCNTs in the starting material, which allowed us to conveniently observe different types of BNNTs on SWCNTs of the identical chirality. We visualize that all zigzag, near-armchair, and chiral BNNTs can form on (6, 5) SWCNTs. As the property of a 1D heterostructure is largely affected by the inner-most template, being able to use atomically precise SWCNT as the starting material is an initial but critical step towards the fabrication of more function-designable 1D hetero-nanotube devices.