Mixed-dimensional Heterojunctions Research Articles

Ab Initio Modeling of Mixed-Dimensional Heterostructures: A Path Forward.

Understanding the electronic structure of mixed-dimensional heterostructures is essential for maximizing their application potential. However, accurately modeling such interfaces is challenging due to the complex interplay between the subsystems. We employ a computational framework integrating first-principles methods, including GW, density functional theory (DFT), and the polarizable continuum model, to elucidate the electronic structure of mixed-dimensional heterojunctions formed by free-base phthalocyanines and monolayer molybdenum disulfide. We assess the impact of dielectric screening across various scenarios, from isolated molecules to organic films on a substrate-supported monolayer. Our findings show that while polarization effects cause significant renormalization of molecular energy levels, band energies and alignments in the most relevant setup can be accurately predicted through DFT simulations of the individual subsystems. Additionally, we analyze orbital hybridization, revealing potential pathways for interfacial charge transfer. This study offers new insights into hybrid inorganic/organic interfaces and provides a practical computational protocol suitable for scaled-up studies.

Electrically Tunable Multiple-Effects Synergistic and Boosted Photoelectric Performance in Te/WSe2 Mixed-Dimensional Heterojunction Phototransistors.

Mix-dimensional heterojunctions (MDHJs) photodetectors (PDs) built from bulk and 2D materials are the research focus to develop hetero-integrated and multifunctional optoelectronic sensor systems. However, it is still an open issue for achieving multiple effects synergistic characteristics to boost sensitivity and enrich the prospect in artificial bionic systems. Herein, electrically tunable Te/WSe2 MDHJs phototransistors are constructed, and an ultralow dark current below 0.1pA and a large on/off rectification ratio of 106 is achieved. Photoconductive, photovoltaic, and photo-thermoelectric conversions are simultaneously demonstrated by tuning the gate and bias. By these synergistic effects, responsivity and detectivity respectively reach 13.9 AW-1 and 1.37×1012 Jones with 400 times increment. The Te/WSe2 MDHJs PDs can function as artificial bionic visual systems due to the comparable response time to those of the human visual system and the presence of transient positive and negative response signals. This work offers an available strategy for intelligent optoelectronic devices with hetero-integration and multifunctions.

Dynamic large-array terahertz imaging display based on high-performance 1D/2D tellurium homojunction modulators

Mixed-dimensional van der Waals systems could improve terahertz modulators’ performance by utilizing the advantages of different dimensional materials. However, the reported available mixed-dimensional heterojunctions using two-dimensional (2D) and three-dimensional materials usually sacrifice the modulation speed to realize a higher modulation depth. Here, we creatively integrate one-dimensional (1D) nanowires with 2D nanofilms to construct the novel mixed-dimensional tellurium (Te) homojunction and achieve optimal indices with an ultrahigh modulation depth and a shorter carrier lifetime. In addition, a Te-based large-array imaging element was fabricated to successfully reproduce the painting colors under specific pump conditions as well as the dynamic multicolor display. Further measurements with the introduction of metamaterials prove that the required energy consumption can be significantly reduced by one order of magnitude. Our proposed 1D/2D integration strategy opens a new way to build high-performance terahertz functional devices and greatly expands the application fields of Te nanomaterials.

Ionic liquid gated black phosphorus/SiC-based 2D/3D mixed-dimensional photodetector

The utilization of 2D layered black phosphorus (BP) in ultraviolet (UV) optoelectronic devices has garnered significant attention due to its high hole mobility and strong light–material interaction. However, its practical applications are impeded by its low light absorption efficiency and environmental instability. To address these challenges, constructing an ionic liquid gated BP/SiC mixed-dimensional photodetector by integrating the high ultraviolet light absorption capacity of SiC with the effective electronic property modulation and surface protection of ionic liquid is suggested in this work. Remarkably, the developed device exhibits an improving photoresponsivity of 10.24 A/W at a gate voltage of −2 V and source–drain voltage of 0.5 V, which is ∼73 times larger than that of the BP UV photodetector, as well as an enhanced environmental stability for at least 30 days. This research offers potential strategies for optimizing the performance of advanced functional devices using 2D/3D mixed-dimensional heterojunctions.

(Invited) Anti-Ambipolar Phototransistors Based on Mixed-Dimensional Heterojunctions

The incapability of modulating the photoresponse of the assembled heterostructure devices has remained a challenge for developing optoelectronics with multi-functionality. In this presentation, we report a gate-tunable and anti-ambipolar phototransistor based on 1D GaAsSb nanowire/2D MoS2 nanoflake mixed-dimensional van der Waals (vdW) heterojunctions. The resulting heterojunction shows asymmetric control over the anti-ambipolar transfer characteristics, possessing the potential to implement electronic functions in logic circuits. Meanwhile, the anti-ambipolar device allows the synchronous adjustment of band slope and depletion regions by gating in both components, thereby giving rise to the gate-tunability of the photoresponse. Coupled with the synergistic effect of the materials in different dimensionality, the hybrid heterojunction can be readily modulated by the external gate to achieve a high-performance photodetector exhibiting a remarkable on/off current ratio of 4×104, fast response of 50 μs and high detectivity of 1.64×1011 Jones. Due to the formation of type-II band alignment and strong interfacial coupling, a prominent photovoltaic response is also explored in the heterojunction. Finally, a visible image sensor based on this hybrid device is demonstrated with excellent imaging capability, suggesting promising applications in future optoelectronic systems.

0D-carbon-quantum-dots/2D-MoS2 mixed-dimensional heterojunction transistor for emulating pulsatile photoelectric therapy of visual amnesic behaviors

0D-carbon-quantum-dots/2D-MoS2 mixed-dimensional heterojunction transistor for emulating pulsatile photoelectric therapy of visual amnesic behaviors

Polarization-sensitive UV photodetector based on ReSe2/GaN mixed-dimensional heterojunction.

Polarization-sensitive photodetectors in the ultraviolet (UV) region have been favored for their great meaning in the field of military and civilian. UV photodetectors based on GaN have aroused much attention due to high photocurrent and high sensitivity. However, the dependence on external power sources and the limited sensitivity to polarized UV light significantly impede the practical application of these photodetectors in UV-polarized photodetection. Herein, a polarization-sensitive UV photodetector based on ReSe2/GaN mixed-dimensional van der Waals (vdWs) heterojunction is proposed. Owing to the high-quality junction and type-II band alignment, the responsivity and specific detectivity reach values of 870 mA/W and 6.8 × 1011 Jones, under 325 nm illumination, respectively. Furthermore, thanks to the strong in-plane anisotropy of ReSe2, the device is highly sensitive to polarized UV light with a photocurrent anisotropic ratio up to 6.67. The findings are expected to bring new opportunities for the development of highly sensitive, high-speed and energy-efficient polarization-sensitive photodetectors.

Electron Transport Properties of Graphene/WS2 Van Der Waals Heterojunctions.

Van der Waals heterojunctions of two-dimensional atomic crystals are widely used to build functional devices due to their excellent optoelectronic properties, which are attracting more and more attention, and various methods have been developed to study their structure and properties. Here, density functional theory combined with the nonequilibrium Green's function technique has been used to calculate the transport properties of graphene/WS2 heterojunctions. It is observed that the formation of heterojunctions does not lead to the opening of the Dirac point of graphene. Instead, the respective band structures of both graphene and WS2 are preserved. Therefore, the heterojunction follows a unique Ohm's law at low bias voltages, despite the presence of a certain rotation angle between the two surfaces within the heterojunction. The transmission spectra, the density of states, and the transmission eigenstate are used to investigate the origin and mechanism of unique linear I-V characteristics. This study provides a theoretical framework for designing mixed-dimensional heterojunction nanoelectronic devices.

Self-powered broadband photodetector based on Bi2Se3/GaN pn mixed-dimensional heterojunction with boosted responsivity

Self-powered broadband photodetectors (SPBDs) have received considerable attention because of their significance in optoelectronic applications. However, limited by problems inherent to the material, such as high dislocation density, complex heteroepitaxy processing of bulk materials, and the small photoabsorption cross section in atomically thin layers of two-dimensional materials, SPBD photodetectors with both a wide detection range and attractive response characteristics are difficult to fabricate. To address this difficulty, herein the synthesis of n-Bi2Se3/p-GaN mixed-dimensional heterojunctions by the transfer method and their assembly into SPBD photodetectors are described. This photodetector offers both a wide spectral response range (ultraviolet to mid-infrared) and excellent response characteristics. The high responsivity (24554 A/W, 3V), remarkable Ion/Ioff (223.8), and rapid rise and decay times (τr = 0.063 s and τd = 0.043 s) benefit from both the built-in electric field of the pn junction and the fast electron transport on the surface of topological insulator Bi2Se3. The results of this research should facilitate the use of this emerging material for the design and fabrication of high-performance SPBD photodetectors.

Van der Waals nanomesh electronics on arbitrary surfaces

Chemical bonds, including covalent and ionic bonds, endow semiconductors with stable electronic configurations but also impose constraints on their synthesis and lattice-mismatched heteroepitaxy. Here, the unique multi-scale van der Waals (vdWs) interactions are explored in one-dimensional tellurium (Te) systems to overcome these restrictions, enabled by the vdWs bonds between Te atomic chains and the spontaneous misfit relaxation at quasi-vdWs interfaces. Wafer-scale Te vdWs nanomeshes composed of self-welding Te nanowires are laterally vapor grown on arbitrary surfaces at a low temperature of 100 °C, bringing greater integration freedoms for enhanced device functionality and broad applicability. The prepared Te vdWs nanomeshes can be patterned at the microscale and exhibit high field-effect hole mobility of 145 cm2/Vs, ultrafast photoresponse below 3 μs in paper-based infrared photodetectors, as well as controllable electronic structure in mixed-dimensional heterojunctions. All these device metrics of Te vdWs nanomesh electronics are promising to meet emerging technological demands.

Borophene-based mixed-dimensional van der waals heterojunctions for high-performance self-powered photodetector

The ongoing pursuit of mixed-dimensional van der Waals heterojunctions has attracted enormous interests owing to their obviously enhanced photoelectrochemical (PEC) characteristics compared to their single components. Herein, we demonstrate the preparation of 0D Bismuth nanoparticles (Bi NPs) coated 2D Borophene (B@Bi) mixed-dimensional heterojunction by hydrothermal method. Benefiting from the promoted photo-induced electron/hole pair separation, shorted charge transfer length, and built-in electric field, the fabricated B@Bi-based PEC-type photodetector (PD) exhibits enhanced self-powered photodetection properties along with the fast response and recovery speed (0.02/0.02 s) in 0.5 M KOH electrolyte under 365 nm illumination. Additionally, repeatable ON/OFF switch signals can be observed after 30 days. Our present work provides an opportunity for fabricating high-performance mixed-dimensional B@Bi-based PDs with good long-term stability.

Charge transfer dynamics and interlayer exciton formation in MoS2/VOPc mixed dimensional heterojunction.

Mixed-dimensional van der Waals heterojunctions involve interfacing materials with different dimensionalities, such as a 2D transition metal dichalcogenide and a 0D organic semiconductor. These heterojunctions have shown unique interfacial properties not found in either individual component. Here, we use femtosecond transient absorption to reveal photoinduced charge transfer and interlayer exciton formation in a mixed-dimensional type-II heterojunction between monolayer MoS2 and vanadyl phthalocyanine (VOPc). Selective excitation of the MoS2 exciton leads to hole transfer from the MoS2 valence band to VOPc highest occupied molecular orbit in ∼710fs. On the contrary, selective photoexcitation of the VOPc layer leads to instantaneous electron transfer from its excited state to the conduction band of MoS2 in less than 100fs. This light-initiated ultrafast separation of electrons and holes across the heterojunction interface leads to the formation of an interlayer exciton. These interlayer excitons formed across the interface lead to longer-lived charge-separated states of up to 2.5ns, longer than in each individual layer of this heterojunction. Thus, the longer charge-separated state along with ultrafast charge transfer times provide promising results for photovoltaic and optoelectronic device applications.

High responsivity photodetector based on mixed-dimensional CsPbBr3/CsPbI2Br heterojunction

• The mixed-dimensional heterojunction PD was fabricated by the solution method. • The heterojunction facilitates the excitons dissociation and carriers transfer. • The PD responsivity is 66 A/W and the specific detectivity is 1.17 × 10 11 Jones. The high-performance photodetector (PD) based on mixed-dimensional (0D/3D) CsPbBr 3 /CsPbI 2 Br heterojunction was fabricated by the solution method. As a sensitizer, CsPbBr 3 can enhance light absorption, and the heterojunction promotes the process of exciton dissociation and carrier transfer, thus improving the comprehensive performance of PD. Under 405 nm irradiation, responsivity and specific detectivity of the device are 66 A/W and 1.17 × 10 11 Jones. This work provides a feasible strategy for heterojunction devices based on perovskite.

Near-UV Phototransistors Based on an All-Inorganic Lead-Free Cs3Cu2I5/CuTCNQ Hierarchical Heterostructure

With the rapid advances in metal halide perovskite optoelectronics, eliminating toxic lead from perovskites has been an urgent demand. However, state-of-the-art lead-free perovskite photodetectors are still challenged with issues of low photoresponse, poor stability, etc. Here, all-inorganic lead-free perovskite (Cs3Cu2I5) single crystals that possess good stability under air exposure are synthesized via a facile solid reaction method. Meanwhile, a higher photoluminescence quantum yield of 95.2% and a prolonged carrier lifetime of 1.127 μs are obtained by further optimizing the synthesis. Benefiting from the polyporous surface and hollow structure of Cu-7,7,8,8-tetracyanoquinodimethane (CuTCNQ) microtubes, more Cs3Cu2I5 nanocrystals can adhere on the innershell and outershell of CuTCNQ-15 microtubes. This unique structure contributes to the improved efficiency of utilizing incident light and promotes charge carrier generation and transportation. As a result, the hierarchical CuTCNQ/Cs3Cu2I5 (hollow microtube/nanocrystal) heterostructure phototransistor exhibits a high responsivity of 88.36 A W-1 and a large detectivity of 1.66 × 1012 Jones. The proposed lead-free perovskites and mixed-dimensional heterojunctions are promising for sensitive light detection.

Ultrahigh photocatalytic hydrogen evolution performance of coupled 1D CdS/1T-phase dominated 2D WS2 nanoheterojunctions

Solar-powered photocatalytic hydrogen production from water using semiconductors provides an eco-friendly and promising approach for converting solar energy into hydrogen fuel. Bulk semiconductors generally suffer from certain limitations, such as poor visible-light utilization, rapid recombination of charge carriers, and low catalytic capability. The key challenge is to develop visible-light-driven heterojunction photocatalysts that are stable and highly active during the water splitting process. Here, we demonstrate the integration of one-dimensional (1D) CdS nanorods with two-dimensional (2D) 1T-phase dominated WS2 nanosheets for constructing mixed-dimensional heterojunctions for the photocatalytic hydrogen evolution reaction (HER). The resulting 1D CdS/2D WS2 nanoheterojunction exhibited an ultrahigh hydrogen-evolution activity of ∼70 mmol·g−1·h−1 that was visible to the naked eye, as well as long-term stability under visible light illumination. The results reveal that the synergy of hybrid nanoarchitectures and intimate interfacial contact between the 1D CdS nanorods and 1T-phase dominated 2D WS2 nanosheets facilitates charge carrier transport, which is beneficial for achieving superior hydrogen evolution.

Range-separated hybrid functionals for mixed dimensional heterojunctions: Application to phthalocyanines/MoS2

We analyze the electronic structure and level alignment of transition-metal phthalocyanine (MPc) molecules adsorbed on two-dimensional MoS2 employing density functional theory (DFT) calculations. We develop a procedure for multi-objective optimal tuning of parameters of range-separated hybrid functionals in these mixed-dimensional systems. Using this procedure, which leads to the asymptotically correct exchange–correlation potential between molecules and two-dimensional materials, we obtain electronic structures consistent with experimental photoemission results for both energy level alignment and electronic bandgaps, representing a significant advance compared to standard DFT methods. We elucidate the MoS2 valence resonance with the transition-metal phthalocyanine non-frontier 3d orbitals and its dependence on the transition-metal atomic number. Based on our calculations, we derive parameter-free model self-energy corrections that quantitatively account for the effects of the heterogeneous dielectric environment on the electronic structure of these mixed-dimensional heterojunctions.

Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment.

The surface sensitivity and lack of dielectric screening in two-dimensional (2D) materials provide numerous intriguing opportunities to tailor their properties using adsorbed π-electron organic molecules. These organic-2D mixed-dimensional heterojunctions are often considered solely in terms of their energy level alignment, i.e., the relative energies of the frontier molecular orbitals versus the 2D material conduction and valence band edges. While this simple model is frequently adequate to describe doping and photoinduced charge transfer, the tools of molecular chemistry enable additional manipulation of properties in organic-2D heterojunctions that are not accessible in other solid-state systems. Fully exploiting these possibilities requires consideration of the details of the organic adlayer beyond its energy level alignment, including hybridization and electrostatics, molecular orientation and thin-film morphology, nonfrontier orbitals and defects, excitonic states, spin, and chirality. This Perspective explores how these relatively overlooked molecular properties offer unique opportunities for tuning optical and electronic characteristics, thereby guiding the rational design of organic-2D mixed-dimensional heterojunctions with emergent properties.

Designing of 0D/2D mixed-dimensional van der waals heterojunction over ultrathin g-C3N4 for high-performance flexible self-powered photodetector

Mixed-dimensional 0D Bi QDs coated 2D g-C 3 N 4 NSs van der Waals heterojunction (g-C 3 N 4 @Bi) is designed and applied for fabricating flexible self-powered photodetector. • Mixed-dimensional 0D Bi QDs coated 2D g-C 3 N 4 (g-C 3 N 4 @Bi) heterojunction is prepared. • g-C 3 N 4 @Bi shows accelerated charge transfer and suppressed carrier recombination. • g-C 3 N 4 @Bi-based flexible photodetector shows self-powered photodetection capability. • The prepared photodetector exhibits good long-term stability (over 90 days). Mixed-dimensional van der Waals heterojunctions have attracted a great deal of attention due to their excellent photoelectric conversion capability, which have been widely applied for self-powered photoelectrochemical (PEC)-type devices as the growing demands of clean energy, environmental governance and protection, and the underwater communications. In this work, 0D Bismuth quantum dots (Bi QDs) encapsulated 2D g-C 3 N 4 nanosheets (g-C 3 N 4 @Bi) mixed-dimensional heterojunctions were prepared and spined on ITO-coated polyethylene terephthalate (PET) substrate for working as self-powered PEC-type photodetector (PD). Density functional theory calculations evidenced the successful construction of a type II van der Waals heterojunction between g-C 3 N 4 and Bi as well as its enhanced optoelectronic characteristics. Benefiting from the superior properties of g-C 3 N 4 @Bi, the as-prepared PD exhibits a high photocurrent of 5.02 μA cm −2 , a responsivity of 2843.14 μA W −1 , and a specific detectivity of 2.25 × 10 11 Jones at 0 V. Furthermore, the PD exhibits self-powered capability after 1000 cycles of bending, and the stable and repeatable ON/OFF signals can be detected over 90 days, indicating the excellent long-term stability and the potential of g-C 3 N 4 @Bi-based PD in practical applications.

Fabrication of 1D Te/2D ReS2 Mixed-Dimensional van der Waals p-n Heterojunction for High-Performance Phototransistor.

The superior optical and electronic properties of the two-dimensional (2D) rhenium disulfide (ReS2) makes it suitable for nanoelectronic and optoelectronic applications. However, the internal defects coupled with with the low mobility and light-absorbing capability of ReS2 impede its utilization in high-performance photodetectors. Fabrication of mixed-dimensional heterojunctions is an alternative method for designing high-performance hybrid photodetectors. This study proposes a mixed-dimensional van der Waals (vdW) heterojunction photodetector, containing high-performance one-dimensional (1D) p-type tellurium (Te) and 2D n-type ReS2, developed by depositing Te nanowires on ReS2 nanoflake using the dry transfer method. It can improve the injection and separation efficiency of photoexcited electron-hole pairs due to the type II p-n heterojunction formed at the ReS2 and Te interface. The proposed heterojunction device is sensitive to visible-light sensitivity (632 nm) with an ultrafast photoresponse (5 ms), high responsivity (180 A/W), and specific detectivity (109), which is superior to the pristine Te and ReS2 photodetectors. As compared to the ReS2 device, the responsivity and response speed is better by an order of magnitude. These results demonstrate the fabrication and application potential of Te/ReS2 mixed-dimensional heterojunction for high-performance optoelectronic devices and sensors.

A defect-induced broadband photodetector based on WS2/pyramid Si 2D/3D mixed-dimensional heterojunction with a light confinement effect.

Broadband photodetection is of vital importance for both civil and technological applications. The widespread use of commercial photodiodes based on traditional semiconductors (e.g. GaN, Si, and InGaAs) is limited to the relatively narrow response range. In this work, we have demonstrated a self-driven and broadband photodetector based on WS2/pyramid Si 2D/3D mixed-dimensional van der Waals (vdW) heterojunction, which is assembled by directly transferring 2D WS2 film on 3D pyramid Si. Thanks to the enhanced light absorption with the pyramid Si structure, the defect-induced narrowed bandgap of the WS2 film, and high-quality vdW heterojunction, impressive device performances in terms of a large responsivity of 290 mA W-1, a high specific detectivity of up to 2.6 × 1014 Jones and an ultrabroad response spectrum ranging from 265 nm to 3.0 μm are achieved at zero bias. Importantly, the photodetector can function as an infrared imaging cell with a high spatial resolution. The totality of these excellent features confirms that the demonstrated WS2/pyramid Si 2D/3D mixed-dimensional vdW heterojunction device may hold great promise for applications in high-performance broadband infrared photodetection and imaging.