Nanoribbon Heterojunctions Research Articles

Topologically enhanced nonlinear optical response of graphene nanoribbon heterojunctions

We study the nonlinear optical properties of heterojunctions made of graphene nanoribbons (GNRs) consisting of two segments with either the same or different topological properties. By utilizing a quantum mechanical approach that incorporates distant-neighbor interactions, we demonstrate that the presence of topological interface states significantly enhances the second- and third-order nonlinear optical response of GNR heterojunctions that are created by merging two topologically inequivalent GNRs. Specifically, GNR heterojunctions with topological interface states display third-order harmonic hyperpolarizabilities that are more than two orders of magnitude larger than those of their similarly sized counterparts without topological interface states, whereas the second-order harmonic hyperpolarizabilities exhibit a more than ten-fold contrast between heterojunctions with and without topological interface states. Additionally, we find that the topological state at the interface between two topologically distinct GNRs can induce a noticeable red-shift of the quantum plasmon frequency of the heterojunctions. Our results reveal a general and profound connection between the existence of topological states and an enhanced nonlinear optical response of graphene nanostructures and possible other photonic systems.

Precision Graphene Nanoribbon Heterojunctions by Chain-Growth Polymerization.

Graphene nanoribbons (GNRs) are considered promising candidates for next-generation nanoelectronics. In particular, GNR heterojunctions have received considerable attention due to their exotic topological electronic phases at the heterointerface. However, strategies for their precision synthesis remain at a nascent stage. Here, we report a novel chain-growth polymerization strategy that allows for constructing GNR heterojunction with N=9 armchair and chevron GNRs segments (9-AGNR/cGNR). The synthesis involves a controlled Suzuki-Miyaura catalyst-transfer polymerization (SCTP) between 2-(6'-bromo-4,4''-ditetradecyl-[1,1':2',1''-terphenyl]-3'-yl) boronic ester (M1) and 2-(7-bromo-9,12-diphenyl-10,11-bis(4-tetradecylphenyl)-triphenylene-2-yl) boronic ester (M2), followed by the Scholl reaction of the obtained block copolymer (poly-M1/M2) with controlled Mn (18k) and narrow Đ (1.45). NMR and SEC analysis of poly-M1/M2 confirm the successful block copolymerization. The solution-mediated cyclodehydrogenation of poly-M1/M2 toward 9-AGNR/cGNR is unambiguously validated by FT-IR, Raman, and UV-vis spectroscopies. Moreover, we also demonstrate the on-surface formation of pristine 9-AGNR/cGNR from the unsubstituted copolymer precursor, which is unambiguously characterized by scanning tunneling microscopy (STM).

Präzise Graphen‐Nanostreifen Heteroübergänge durch Kettenwachstumspolymerisation

AbstractGraphene‐Nanostreifen (GNS, engl. GNR) werden als vielversprechende Kandidaten für Nanoelektroniken der nächsten Generation gehandelt. Besonders Heteroübergänge in diesen Materialien erhalten durch ihre exotischen topologischen elektronischen Phasen am Heteroübergang in letzter Zeit viel Aufmerksamkeit. Allerdings sind Synthesestrategien für die atompräzise Synthese noch am Beginn ihrer Entwicklung. Hier berichten wir von einer neuartigen Strategie zur Kettenwachstumspolymerisation für die Konstruktion von Heteroübergängen zwischen N=9 Sessel‐GNR (engl. armchair, 9‐AGNR) und chevron GNRs (cGNR). Die Synthese beinhaltet eine kontrollierte Suzuki–Miyaura Katalysatortransferpolymerisation (engl. SCTP) zwischen 2‐(6′‐bromo‐4,4′′‐ditetradecyl‐[1,1′:2′,1′′‐terphenyl]‐3′‐yl)boronsäureester (M1) und 2‐(7‐bromo‐9,12‐diphenyl‐10,11‐bis(4‐tetradecylphenyl)‐triphenylen‐2‐yl)boronsäureester (M2) um das Blockcopolymer poly‐M1/M2 mit einem kontrollierten Mn (18 kDa) und einem schmalen Polydispersitätsindex Đ (1.45) zu erhalten. Darauf folgt eine Schollreaktion des Blockcopolymers zur Heterostruktur 9‐AGNR/cGNR. NMR und SEC von poly‐M1/M2 bestätigen die erfolgreiche Blockcopolymerisation. Die lösungsvermittelte Zyklodehydrierung von poly‐M1/M2 zu 9‐AGNR/cGNR wurde eindeutig über FT‐IR, Raman und UV/Vis Spektroskopie nachgewiesen. Weiter konnten wir die Ausbildung von 9‐AGNR/cGNR durch Oberflächenchemie des unsubstituierten Blockcopolymers mit Hilfe von Rastertunnelmikroskopie (RTM) direkt visualisieren.

Quasiparticles in Graphene Nanoribbon Heterojunctions: Insights from a Simplified One-Dimensional Model

Quasiparticles in Graphene Nanoribbon Heterojunctions: Insights from a Simplified One-Dimensional Model

Photoresponse of Solution-Synthesized Graphene Nanoribbon Heterojunctions on Diamond Indicating Phototunable Photodiode Polarity

Graphene nanoribbon heterostructures and heterojunctions have attracted interest as next-generation molecular diodes with atomic precision. Their mass production via solution methods and prototypical device integration remains to be explored. Here, the bottom-up solution synthesis and characterization of liquid-phase-processable graphene nanoribbon heterostructures (GNRHs) are demonstrated. Joint photoresponsivity measurements and simulations provide evidence of the structurally defined heterostructure motif acting as a type-I heterojunction. Real-time, time-dependent density functional tight-binding simulations further reveal that the photocurrent polarity can be tuned at different excitation wavelengths. Our results introduce liquid-phase-processable, self-assembled heterojunctions for the development of nanoscale diode circuitry and adaptive hardware.

Facile synthesis of graphene quantum dots and C-doping porous BN nanoribbon heterojunctions for boosting CO2 photoreduction

Herein, the graphene quantum dots (GQDs) as a photosensitizer have been introduced on the surface of C-doping porous BN nanoribbons (C-BNNR) to form a series of 0D/1D GQDs/C-BNNR heterojunctions as metal-free catalysts with excellent photocatalytic activity. the C-BNNR with sensitization of GQDs is innovatively applied to the field of CO2 photocatalytic reduction. The results reveal that GQDs/C-BNNR heterojunctions exhibit excellent optical properties and photocatalytic activity. The CO production rate of GQDs/C-BNNR-3 (33.47 μmol g-1h-1) is 14.49, 4.60 and 2.95 times higher than GQDs (2.31 μmol g-1h-1), BNNR (7.27 μmol g-1h-1) and C-BNNR (11.34 μmol g-1h-1), respectively. This work demonstrates that the addition of GQDs can significantly improve the light utilization of C-BNNR without any effect on the structure. In addition, the strong interaction between the GQDs and C-BNNR leads to the formation of a built-in electric field. It allows photogenerated electrons in GQDs to be transferred into C-BNNR and complexed with holes. As a result, the photogenerated electrons in C-BNNR are induced to participate in the CO2 photocatalytic reduction process. Meanwhile, GQDs/C-BNNR possess certain CO2 photocatalytic reduction ability in the NIR region due to the upconversion ability of GQDs. This work pioneer the application of GQDs/C-BNNR heterojunctions in CO2 photocatalytic reduction and further provides new ideas for the design of novel metal-free catalysts.

Spin-Polarized Resonant Tunneling in Antiferromagnetic Heterojunctions of Graphene Nanoribbons with 3d Adatoms

Binding, magnetic, and electron transport properties of $3d$ transition-metal (TM) adatoms ($\mathrm{Cr}$-$\mathrm{Cu}$) on an antiferromagnetic armchair graphene nanoribbon heterojunction are studied by means of density-functional-theory (DFT) calculations and nonequilibrium Green function (NEGF) formalism. The heterojunction emulates a system of two symmetric potential barriers and one well with resonances (quasibound states). Binding energies show a strong interaction between $3d$ TM adatoms and heterojunction hollow sites, where $3d$ TM adatoms prefer the edges rather than the middle of the graphene heterojunction. As for magnetic properties, heterojunctions with Cr, $\mathrm{Mn}$, and $\mathrm{Fe}$ have a ferromagnetic behavior with a total magnetic moment $\ensuremath{\ge}4{\ensuremath{\mu}}_{B}$, while $\mathrm{Co}$, $\mathrm{Ni}$, and $\mathrm{Cu}$ adatoms induce antiferromagnetism in the full system with total magnetizations $<3{\ensuremath{\mu}}_{B}$. Transmission curves show that electronic resonances undergo a spin-splitting effect induced by $3d$ TM adatoms, and current versus bias-voltage curves show spin currents from which spin-polarized ratios (SPRs) $>75\mathrm{%}$ for $\mathrm{Co}$ and approximately $100\mathrm{%}$ for $\mathrm{Ni}$ adatoms are calculated. Finally, an oscillatory behavior is observed in SPR curves at low bias due to a combined effect of resonant tunneling and spin filtering. Therefore, these results suggest that the present antiferromagnetic graphene heterojunction with $\mathrm{Co}$ and $\mathrm{Ni}$ adatoms is suitable for future spin and low-power nanoelectronics as a spin-dependent resonant tunneling device.

Vertical quantum tunneling transport based on MoS2/WTe2 nanoribbons

The quantum electrical and thermal transport properties of band-to-band tunneling are studied in the van der Waals (vdW) vertical MoS2/WTe2 nanoribbon heterojunction as well as the lateral MoS2/WTe2 heterojunction. The computational method is based on the Green's function method within the tight-binding approach in the coherent regime. The numerical results show distinct properties, such as a noticeable rectification ratio (RR) and a negative differential resistance (NDR). This device can act as a vertical tunneling transistor structure. Besides, the MoS2/WTe2 nanoribbon devices with the armchair termination exhibit the highest value of the ZTe at μ=±0.72 eV leading to their improved thermoelectric properties. Our findings about the hybrid heterostructures thus shed a new light toward extend the applications of 2D monolayer transition metal dichalcogenides (TDMs) materials in electronic and optoelectronic devices.

Multifunctional spintronic device based on zigzag SiC nanoribbon heterojunction via edge asymmetric dual-hydrogenation

The authors propose a method to obtain the multifunctional spintronic device by investigating the spin-resolved transport characteristics of zigzag SiC nanoribbon (zSiCNR) heterojunction via edge asymmetric dual-hydrogenation. The spin-resolved band structures show that the dual-hydrogenation on edge C or Si atoms all can change the initial metallicity of the pristine zSiCNR with the edge mono-hydrogenation to semiconductor in the presence of ferromagnetic field. The spin-resolved current-voltage characteristics of the zSiCNR heterojunction can show spin current rectification in the same rectify direction under the parallel magnetic field. The up-spin rectification ratio of our junction can be close to 10 13 at −0.5 V, which is much larger than rectification ratios of the previous junctions induced by the anti-parallel magnetic field. The zSiCNR heterojunction also exhibits a perfect spin filtering behavior with 100% spin filtering efficiency in both positive and negative bias regions under the anti-parallel magnetic field. More interestingly, the magnetoresistance is achieved by manipulating the external magnetic field in our zSiCNR heterojunction with the giant magnetoresistance ratio 5000% at 0.5 V. Therefore, the zSiCNR heterojunction via edge asymmetric dual-hydrogenation can be designed into the multifunctional spintronic devices, which has broad application prospects in the field of future spintronics. • Zigzag silicon carbon nanoribbon has potential in spin devices. • Giant spin current rectification is obtained in the presence of a ferromagnetic field. • Maximum spin current rectification ratios is closing to 10 13 . • Perfect spin filtering and magnetoresistance behaviors are also obtained.

π -magnetism and spin-dependent transport in boron pair doped armchair graphene nanoribbons

Carbon-based magnetic nanostructures have long spin coherent length and are promising for spintronics applications in data storage and information processing. Recent experiments demonstrate that a pair of substitutional boron atoms (B2) doped 7-atom-wide armchair graphene nanoribbons (B2-7AGNRs) have intrinsic magnetism, providing a quasi-1D magnetic material platform for spintronics. In this work, we demonstrate that the magnetism in B2-7AGNRs is contributed by π-electrons, originating from the imbalance of electrons in two spin channels in response to boron dopants. The spin-dependent transport across single and double boron pair doped 7AGNRs (B2-7AGNRs and 2B2-7AGNRs) by constructing lateral graphene nanoribbon heterojunctions has been investigated by using first-principles calculations. We show that for B2-7AGNRs with spin splitting π-electronic states near the Fermi level, by applying a bias voltage, one can obtain a current spin polarization over 90% and a negative differential resistance effect. For 2B2-7AGNRs, two spin centers have been found to be antiferromagnetically coupled. We demonstrate a magnetoresistance effect over 15 000% by setting those two spin centers to be ferromagnetic and antiferromagnetic alignments. Based on the above spin-polarized transport properties, we reveal that GNR heterojunctions based on B2-7AGNRs could be potentially applied in quasi-1D spintronic devices.

A Waveguide-Integrated Two-Dimensional Light-Emitting Diode Based on p-Type WSe2/n-Type CdS Nanoribbon Heterojunction.

Transition metal dichalcogenides (TMDs) have emerged as two-dimensional (2D) building blocks to construct nanoscale light sources. To date, a wide array of TMD-based light-emitting devices (LEDs) have been successfully demonstrated. Yet, their atomically thin and planar nature entails an additional waveguide/microcavity for effective optical routing/confinement. In this sense, integration of TMDs with electronically active photonic nanostructures to form a functional heterojunction is of crucial importance for 2D optoelectronic chips with reduced footprint and higher integration capacity. Here, we report a room-temperature waveguide-integrated light-emitting device based on a p-type monolayer (ML) tungsten diselenide (WSe2) and n-type cadmium sulfide (CdS) nanoribbon (NR) heterojunction diode. The hybrid LED exhibited clear rectification under forward biasing, giving pronounced electroluminescence (EL) at 1.65 eV from exciton resonances in ML WSe2. The integrated EL intensity against the driving current shows a superlinear profile at a high current level, implying a facilitated carrier injection via intervalley scattering. By leveraging CdS NR waveguides, the WSe2 EL can be efficiently coupled and further routed for potential optical interconnect functionalities. Our results manifest the waveguided LEDs as a dual-role module for TMD-based optoelectronic circuitries.

Edge modified phosphorene nanoribbon heterojunctions: promising metal-free photocatalysts for direct overall water splitting

Edge modified phosphorene nanoribbon heterojunctions: promising metal-free photocatalysts for direct overall water splitting

Diffusion-controlled on-surface synthesis of graphene nanoribbon heterojunctions.

We report a new diffusion-controlled on-surface synthesis approach for graphene nanoribbons (GNR) consisting of two types of precursor molecules, which exploits distinct differences in the surface mobilities of the precursors. This approach is a step towards a more controlled fabrication of complex GNR heterostructures and should be applicable to the on-surface synthesis of a variety of GNR heterojunctions.

Topologically protected edge and confined states in finite armchair graphene nanoribbons and their junctions

Topologically protected edge and junction states, previously predicted, have recently been observed in armchair graphene nanoribbon (AGNR) heterojunctions. Here, via tight-binding-based calculations, we explain the relation between the nature and number of the zero-mode edge states of finite-length AGNRs and their structure, topological invariants, and winding number. This allows us to rationalize the design of AGNR heterojunctions and superlattices with tailored phases. We show how the choice of widths, interface coupling geometry, and boundaries determines the emergence of topological states following patterns that depend on the structure and family of the constituent AGNRs. Furthermore, we prove that quantum-well-like states confined in one of the constituent ribbons develop in all the AGNR junctions irrespective of their trivial or topological character. The bipartite nature of the honeycomb lattice is determinant for the topological properties of the junctions: their electronic states can be topologically trivial or nontrivial depending on subtle differences at the boundaries of the ribbons.

Electronic Structure and Transport in Graphene Nanoribbon Heterojunctions under Uniaxial Strain: Implications for Flexible Electronics

Graphene nanoribbons (GNRs) are highly tunable electronic materials that can be assembled to form two-dimensional (2D) nanoarchitectures with unique electronic features. In this paper, we investigate 1D and 2D GNR-based heterojunctions by combining chevron and 9-armchair GNRs together. We then investigate the electronic structure and transport of GNR heterojunctions using first-principles simulations. The coupling of chevron and 9-armchair GNRs leads to the formation of emergent electronic states at the interface due to the geometric mismatch. These states strongly control the electronic structure of the heterojunction. Doping the heterojunction with nitrogen on the edges of the chevron segment shows that these emergent states could disappear from the lowest unoccupied molecular orbital due to the band alignment of the connected GNRs. We also investigate the effect of applying uniaxial strain on the electronic characteristics of the 1D and 2D nanoarchitectures. The results show a lower bandgap and higher electronic conductance as the compressive strain is increased. The nitrogen-doped 2D structure exhibits a lower bandgap and higher electronic currents due to higher transmission pathways. Overall, these fundamental insights could help in the development of 2D GNR-based flexible and wearable devices.

Tunneling current modulation in atomically precise graphene nanoribbon heterojunctions

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.

Rectification with controllable directions in sulfur-doped armchair graphene nanoribbon heterojunctions

By employing nonequilibrium Green’s function methods in combination with density functional theory, we investigate electronic structures and transport properties of the armchair graphene nanoribbon (AGNR) heterojunctions consisting of the sulfur(S)-doped AGNR and the pristine one, which are realizable in experiments. The results show that such heterojunctions own inherent rectifying behaviour with a high rectifying ratio up to 10 4 , and the rectifying direction is totally controllable by choosing the width of ribbon. It is revealed that the rectifying mechanism is driven by easily pinning valence-band maximum (VBM) on the conduction band minimum (CBM) under forward biases but hardly pinning under reverse biases, and the rectifying direction is described by the interrelationship of the band gap between the S-doped AGNR and AGNR segments. Our findings provide a route for the design of AGNR-based rectifiers with controllable direction in molectronics.

Fabrication of a MoS2/Graphene Nanoribbon Heterojunction Network for Improved Thermoelectric Properties

AbstractA number of 2D materials have been developed that have properties different from bulk materials due to the quantum confinement effect. These 2D materials can also form a vertical van der Waals heterojunction at a large interface with other 2D materials, resulting in unique electronic and thermoelectric properties. However, it is difficult to fabricate a van der Waals heterostructure of 2D materials that can provide a sufficient temperature gradient while also forming carrier paths across the vertical heterojunction. Here, a heterojunction network structure constructed of highly conductive sub‐20‐nm graphene nanoribbons (GNRs) arrays stacked on a semiconducting molybdenum disulfide (MoS2) monolayer is suggested to maximize the heterojunction effect on carrier transport. This heterojunction network allows the carriers inevitably pass back and forth between the GNR and the MoS2 through the vertical heterojunction, effectively utilizing the interfacial properties in thermoelectricity. This structure also can modify the band structure by controlling the linewidth of the nanoribbons, or introduce an interlayer at the heterojunctions to enhance the tunneling effect between the two layers, thus significantly improved thermoelectric properties are achieved such as enhanced electrical conductivity of 700 S m−1 and a high power factor of 222 µW m−1 K−1.

Wave-Function Symmetry Mechanism of Quantum-Well States in Graphene Nanoribbon Heterojunctions

The construction of quantum wells (QWs) in graphene nanoribbons is interesting for applications in optoelectronics and quantum information processing, but is hindered by insufficient understanding of quantum mechanical effects in these systems. This study uses theory to reveal that, unexpectedly, electronic states confined by higher confinement energy exhibit $w\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}k\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r$ quantum confinement, due to the wave-function symmetries of the electronic states from the constituent graphene nanoribbon segments. This mechanism leads to a design method for such QWs via modulation of geometric parameters and displacement between constituent nanoribbon segments.

Rectification in zigzag graphene/BN nanoribbon heterojunction

We investigate the effect of changed BN nanoribbon on the rectifying behavior in zigzag graphene/BN nanoribbon heterojunction using first principles based on non-equilibrium Green’s function and density functional theory. The increased BN length in the scattering region reduces the rectifying performance of the device, and the maximum rectifying ratio is [Formula: see text] in the heterojunction. We discuss the different rectifying characteristics for the designed models by calculating the transmission spectra at different biases. The rectifying phenomenon is further investigated by the projected density of state of device. Furthermore, we explain the observed negative differential resistance effect by the transmission spectra and transmission eigenstates. The results suggest that the zigzag graphene/BN nanoribbon heterojunction leads to the asymmetric current, causing the rectifying phenomenon, and the BN length in the scattering region can modulate the rectifying performance of zigzag graphene/BN nanoribbon heterojunction.