Enhancing 2D Photonics and Optoelectronics with Artificial Microstructures.

By modulating subwavelength structures and integrating functional materials, 2D artificial microstructures (2D AMs), including heterostructures, superlattices, metasurfaces and microcavities, offer a powerful platform for significant manipulation of light fields and functions. These structures hold great promise in high-performance and highly integrated optoelectronic devices. However, a comprehensive summary of 2D AMs remains elusive for photonics and optoelectronics. This review focuses on the latest breakthroughs in 2D AM devices, categorized into electronic devices, photonic devices, and optoelectronic devices. The control of electronic and optical properties through tuning twisted angles is discussed. Some typical strategies that enhance light-matter interactions are introduced, covering the integration of 2D materials with external photonic structures and intrinsic polaritonic resonances. Additionally, the influences of external stimuli, such as vertical electric fields, enhanced optical fields and plasmonic confinements, on optoelectronic properties is analysed. The integrations of these devices are also thoroughly addressed. Challenges and future perspectives are summarized to stimulate research and development of 2D AMs for future photonics and optoelectronics.

Discovery of Type II Interlayer Trions.

In terms of interlayer trions, electronic excitations in van der Waals heterostructures (vdWHs) can be classified into Type I (i.e., two identical charges in the same layer) and Type II (i.e., two identical charges in the different layers). Type I interlayer trions are investigated theoretically and experimentally. By contrast, Type II interlayer trions remain elusive in vdWHs, due to inadequate free charges, unsuitable band alignment, reduced Coulomb interactions, poor interface quality, etc. Here, the first observation of Type II interlayer trions is reported by exploring band alignments and choosing an atomically thin organic-inorganic system-monolayer WSe2 /bilayer pentacene heterostructure (1L + 2L HS). Both positive and negative Type II interlayer trions are electrically tuned and observed via PL spectroscopy. In particular, Type II interlayer trions exhibit in-plane anisotropic emission, possibly caused by their unique spatial structure and anisotropic charge interactions, which is highly correlated with the transition dipole moment of pentacene. The results pave the way to develop excitonic devices and all-optical circuits using atomically thin organic-inorganic bilayers.

Ultrafast and Sensitive Self-Powered Photodetector Based on Graphene/Pentacene Single Crystal Heterostructure with Weak Light Detection Capacity.

Organic materials exhibit efficient light absorption and low-temperature, large-scale processability, and have stimulated enormous research efforts for next-generation optoelectronics. While, high-performance organic devices with fast speed and high responsivity still face intractable challenges, due to their intrinsic limitations including finite carrier mobility and high exciton binding energy. Here an ultrafast and highly sensitive broadband phototransistor is demonstrated by integrating high-quality pentacene single crystal with monolayer graphene. Encouragingly, the -3 dB bandwidth can reach up to 26 kHz, which is a record-speed for such sensitized organic phototransistors. Enormous absorption, long exciton diffusion length of pentacene crystal, and efficient interfacial charge transfer enable a high responsivity of >105  A W-1  and specific detectivity of >1011  Jones. Moreover, self-powered weak-light detection is realized using a simple asymmetric configuration, and the obvious zero-bias photoresponses can be displayed even under 750 nW cm-2  light intensity. Excellent response speed and photoresponsivity enable high-speed image sensor capability in UV-Vis ranges.  The results offer a practical strategy for constructing high-performance self-powered organic hybrid photodetectors, with strong applicability in wireless, weak-light detection, and video-frame-rate imaging applications.

Engineering the Dynamics and Transport of Excitons, Trions, and Biexcitons in Monolayer WS2.

The study of transport and diffusion dynamics of quasi-particles such as excitons, trions, and biexcitons in two-dimensional (2D) semiconductors has opened avenues for their application in high-speed excitonic and optoelectronic devices. However, long-range transport and fast diffusion of these quasi-particles have not been reported for 2D systems such as transition metal dichalcogenides (TMDCs). The reported diffusion coefficients from TMDCs are low, limiting their use in high-speed excitonic devices and other optoelectronic applications. Here, we report the highest exciton diffusion coefficient value in monolayer WS2 achieved via engineering the radiative lifetime and diffusion lengths using static back-gate voltage and substrate engineering. Electrostatic doping is observed to modulate the radiative lifetime and in turn the diffusion coefficient of excitons by ∼three times at room temperature. By combining electrostatic doping and substrate engineering, we push the diffusion coefficient to an extremely high value of 86.5 cm2/s, which has not been reported before in TMDCs and is even higher than the values in some 1D systems. At low temperatures, we further report the control of dynamic and spatial diffusion of excitons, trions, and biexcitons from WS2. The electrostatic control of dynamics and transport of these quasi-particles in monolayers establishes monolayer TMDCs as ideal candidates for high-speed excitonic circuits, optoelectronic, and photonic device applications.

Direct observation of contact resistivity for monolayer TMD based junctions via PL spectroscopy.

Monolayer transition metal dichalcogenides (mTMDs) possess a direct band gap and strong PL emission that is highly sensitive to doping level and interfaces, laying the foundation for investigating the contact between mTMD and metal via PL spectroscopy. Currently, electrical methods have been utilized to measure the contact resistance (RC), but they are complicated, time-consuming, high-cost and suffer from inevitable chemical disorders and Fermi level pinning. In addition, previously reported contact resistances comprise both Schottky barrier and tunnel barrier components. Here, we report a simple, rapid and low-cost method to study the tunnel barrier dominated contact resistance of mTMD based junctions through PL spectroscopy. These junctions are free from chemical disorders and Fermi level pinning. Excluding the Schottky barrier component, solely tunnel barrier dominated contact resistances of 1 L MoSe2/Au and 1 L MoSe2/graphene junctions were estimated to be 147.8 Ω µm and 54.9 Ω µm, respectively. Density functional theory (DFT) simulations revealed that the larger RC of the former was possibly due to the existence of intrinsic effective potential difference (Φbarrier) between mTMD and metal. Both junctions exhibit an increasing tendency of RC as temperature decreases, which is probably attributed to the thermal expansion coefficient (TEC) mismatch-triggered interlayer spacing (d) increase and temperature-induced doping. Remarkably, a significant change of RC was observed in 1 L MoSe2/Au junctions, which is possibly ascribed to the changes of their orbital overlaps. Our results open new avenues for exploring fundamental metal-semiconductor contact principles and constructing high-performance devices.

2D Materials and Heterostructures at Extreme Pressure.

2D materials possess wide-tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure-property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high-pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent-organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure-induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward.

Direct Measurement of Folding Angle and Strain Vector in Atomically Thin WS2 Using Second-Harmonic Generation.

Structural engineering techniques such as local strain engineering and folding provide functional control over critical optoelectronic properties of 2D materials. Local strain engineering at the nanoscale level is practically achieved via permanently deformed wrinkled nanostructures, which are reported to show photoluminescence enhancement, bandgap modulation, and funneling effect. Folding in 2D materials is reported to tune optoelecronic properties via folding angle dependent interlayer coupling and symmetry variation. The accurate and efficient monitoring of local strain vector and folding angle is important to optimize the performance of optoelectronic devices. Conventionally, the accurate measurement of both strain amplitude and strain direction in wrinkled nanostructures requires the combined usage of multiple tools resulting in manufacturing lead time and cost. Here, we demonstrate the usage of a single tool, polarization-dependent second-harmonic generation (SHG), to determine the folding angle and strain vector accurately and efficiently in ultrathin WS2. The folding angle in trilayer WS2 folds exhibiting 1-9 times SHG enhancement is probed through variable approaches such as SHG enhancement factor, maxima and minima SHG phase difference, and linear dichroism. In compressive strain induced wrinkled nanostructures, strain-dependent SHG quenching and enhancement is observed parallel and perpendicular, respectively, to the direction of the compressive strain vector, allowing us to determine the local strain vector accurately using a photoelastic approach. We further demonstrate that SHG is highly sensitive to band-nesting-induced transition (C-peak), which can be significantly modulated by strain. Our results show SHG as a powerful probe to folding angle and strain vector.

Towards future physics and applications via two-dimensional material NEMS resonators.

Two-dimensional materials (2Dm) offer a unique insight into the world of quantum mechanics including van der Waals (vdWs) interactions, exciton dynamics and various other nanoscale phenomena. 2Dm are a growing family consisting of graphene, hexagonal-Boron Nitride (h-BN), transition metal dichalcogenides (TMDs), monochalcogenides (MNs), black phosphorus (BP), MXenes and 2D organic crystals such as small molecules (e.g., pentacene, C8 BTBT, perylene derivatives, etc.) and polymers (e.g., COF and MOF, etc.). They exhibit unique mechanical, electrical, optical and optoelectronic properties that are highly enhanced as the surface to volume ratio increases, resulting from the transition of bulk to the few- to mono- layer limit. Such unique attributes include the manifestation of highly tuneable bandgap semiconductors, reduced dielectric screening, highly enhanced many body interactions, the ability to withstand high strains, ferromagnetism, piezoelectric and flexoelectric effects. Using 2Dm for mechanical resonators has become a promising field in nanoelectromechanical systems (NEMS) for applications involving sensors and condensed matter physics investigations. 2Dm NEMS resonators react with their environment, exhibit highly nonlinear behaviour from tension induced stiffening effects and couple different physics domains. The small size and high stiffness of these devices possess the potential of highly enhanced force sensitivities for measuring a wide variety of un-investigated physical forces. This review highlights current research in 2Dm NEMS resonators from fundamental physics and an applications standpoint, as well as presenting future possibilities using these devices.

Supertransport of excitons in atomically thin organic semiconductors at the 2D quantum limit.

Long-range and fast transport of coherent excitons is important for the development of high-speed excitonic circuits and quantum computing applications. However, most of these coherent excitons have only been observed in some low-dimensional semiconductors when coupled with cavities, as there are large inhomogeneous broadening and dephasing effects on the transport of excitons in their native states in materials. Here, by confining coherent excitons at the 2D quantum limit, we first observed molecular aggregation-enabled 'supertransport' of excitons in atomically thin two-dimensional (2D) organic semiconductors between coherent states, with a measured high effective exciton diffusion coefficient of ~346.9 cm2/s at room temperature. This value is one to several orders of magnitude higher than the values reported for other organic molecular aggregates and low-dimensional inorganic materials. Without coupling to any optical cavities, the monolayer pentacene sample, a very clean 2D quantum system (~1.2 nm thick) with high crystallinity (J-type aggregation) and minimal interfacial states, showed superradiant emission from Frenkel excitons, which was experimentally confirmed by the temperature-dependent photoluminescence (PL) emission, highly enhanced radiative decay rate, significantly narrowed PL peak width and strongly directional in-plane emission. The coherence in monolayer pentacene samples was observed to be delocalised over ~135 molecules, which is significantly larger than the values (a few molecules) observed for other organic thin films. In addition, the supertransport of excitons in monolayer pentacene samples showed highly anisotropic behaviour. Our results pave the way for the development of future high-speed excitonic circuits, fast OLEDs, and other optoelectronic devices.

Quantifying Quasi-Fermi Level Splitting and Mapping its Heterogeneity in Atomically Thin Transition Metal Dichalcogenides.

One of the most fundamental parameters of any photovoltaic material is its quasi-Fermi level splitting (∆µ) under illumination. This quantity represents the maximum open-circuit voltage (Voc ) that a solar cell fabricated from that material can achieve. Herein, a contactless, nondestructive method to quantify this parameter for atomically thin 2D transition metal dichalcogenides (TMDs) is reported. The technique is applied to quantify the upper limits of Voc that can possibly be achieved from monolayer WS2 , MoS2 , WSe2 , and MoSe2 -based solar cells, and they are compared with state-of-the-art perovskites. These results show that Voc values of ≈1.4, ≈1.12, ≈1.06, and ≈0.93 V can be potentially achieved from solar cells fabricated from WS2 , MoS2 , WSe2 , and MoSe2 monolayers at 1 Sun illumination, respectively. It is also observed that ∆µ is inhomogeneous across different regions of these monolayers. Moreover, it is attempted to engineer the observed ∆µ heterogeneity by electrically gating the TMD monolayers in a metal-oxide-semiconductor structure that effectively changes the doping level of the monolayers electrostatically and improves their ∆µ heterogeneity. The values of ∆µ determined from this work reveal the potential of atomically thin TMDs for high-voltage, ultralight, flexible, and eye-transparent future solar cells.

Ferroelectric-Driven Exciton and Trion Modulation in Monolayer Molybdenum and Tungsten Diselenides.

In this work, we show how domain-engineered lithium niobate can be used to selectively dope monolayer molybdenum selenide (MoSe2) and tungsten selenide (WSe2) and demonstrate that these ferroelectric domains can significantly enhance or inhibit photoluminescence (PL), with the most dramatic modulation occurring at the heterojunction interface between two domains. A micro-PL and Raman system is used to obtain spatially resolved images of the differently doped transition metal dichalcogenides (TMDs). The domain-inverted lithium niobate causes changes in the TMDs due to electrostatic doping as a result of the remnant polarization from the substrate. Moreover, the differently doped TMDs (n-type MoSe2 and p-type WSe2) exhibit opposite PL modulation. Distinct oppositely charged domains were obtained with a 9-fold PL enhancement for the same single MoSe2 sheet when adhered to the positive (P+) and negative (P-) domains. This sharp PL modulation on the ferroelectric domain results from different free electron or hole concentrations in the material's conduction band or valence band. Moreover, excitons dissociate rapidly at the interface between the P+ and P- domains due to the built-in electric field. We are able to adjust the charge on the P+ and P- domains using temperature via the pyroelectric effect and observe rapid PL quenching over a narrow temperature range, illustrating the observed PL modulation is electronic in nature. This observation creates an opportunity to harness the direct bandgap TMD 2D materials as an active optical component for the lithium niobate platform using domain engineering of the lithium niobate substrate to create optically active heterostructures that could be used for photodetectors or even electrically driven optical sources on-chip.

Radiation tolerance of two-dimensional material-based devices for space applications.

Characteristic for devices based on two-dimensional materials are their low size, weight and power requirements. This makes them advantageous for use in space instrumentation, including photovoltaics, batteries, electronics, sensors and light sources for long-distance quantum communication. Here we present a comprehensive study on combined radiation effects in Earth's atmosphere on various devices based on these nanomaterials. Using theoretical modeling packages, we estimate relevant radiation levels and then expose field-effect transistors, single-photon sources and monolayers as building blocks for future electronics to γ-rays, protons and electrons. The devices show negligible change in performance after the irradiation, suggesting robust suitability for space use. Under excessive γ-radiation, however, monolayer WS2 shows decreased defect densities, identified by an increase in photoluminescence, carrier lifetime and a change in doping ratio proportional to the photon flux. The underlying mechanism is traced back to radiation-induced defect healing, wherein dissociated oxygen passivates sulfur vacancies.

Modulated interlayer charge transfer dynamics in a monolayer TMD/metal junction.

The performance of optoelectronic devices based on monolayer transition-metal dichalcogenide (mTMD) semiconductors is significantly affected by the contact at the mTMD-metal interface, which is dependent on interlayer interactions and coupling. Here, we report a systematic optical method to investigate the interlayer charge transfer and coupling in a mTMD-metal heterojunction. Giant photoluminescence (PL) quenching was observed in a monolayer MoS2/Pd (1L MoS2/Pd) junction which is mainly due to the efficient interlayer charge transfer between Pd and MoS2. 1L MoS2/Pd also exhibits an increase in the PL quenching factor (η) as the temperature decreases, due to a reduction of the interlayer spacing. Annealing experiments were also performed which supported interlayer charge transfer as the main mechanism for the increase of η. Moreover, a monolayer MoS2/Au (1L MoS2/Au) junction was fabricated for engineering the interlayer charge transfer. Interestingly, a narrowing effect of the full width at half maximum (FWHM) was encountered as the junctions changed from 1L MoS2/SiO2 → 1L MoS2/Au → 1L MoS2/Pd, possibly originating from a change of the doping level induced weakening of exciton-carrier scattering. Our results deepen the understanding of metal-semiconductor junctions for further exploring fundamental phenomena and enabling high-performance devices using mTMD-metal junctions.

High-Efficiency Monolayer Molybdenum Ditelluride Light-Emitting Diode and Photodetector.

Developing a high-efficiency and low-cost light source with emission wavelength transparent to silicon is an essential step toward silicon-based nanophotonic devices and micro/nano industry platforms. Here, a near-infrared monolayer MoTe2 light-emitting diode (LED) has been demonstrated and its emission wavelength is transparent to silicon. By taking advantage of the quantum tunneling effect, the device has achieved a very high external quantum efficiency (EQE) of 9.5% at 83 K, which is the highest EQE obtained from LED devices fabricated from monolayer TMDs so far. When the device is operated as a photodetector, the MoTe2 device exhibits a strong photoresponsivity at resonant wavelength 1145 nm. The low dark current of ∼5pA and fast response time 5.06 ms are achieved due to suppression of hBN tunneling layer. Our results open a new route for the investigation of novel near-infrared silicon integrated optoelectronic devices.

Efficient and Layer-Dependent Exciton Pumping across Atomically Thin Organic-Inorganic Type-I Heterostructures.

The fundamental light-matter interactions in monolayer transition metal dichalcogenides might be significantly engineered by hybridization with their organic counterparts, enabling intriguing optoelectronic applications. Here, atomically thin organic-inorganic (O-I) heterostructures, comprising monolayer MoSe2 and mono-/few-layer single-crystal pentacene samples, are fabricated. These heterostructures show type-I band alignments, allowing efficient and layer-dependent exciton pumping across the O-I interfaces. The interfacial exciton pumping has much higher efficiency (>86 times) than the photoexcitation process in MoSe2 , although the pentacene layer has much lower optical absorption than MoSe2 . This highly enhanced pumping efficiency is attributed to the high quantum yield in pentacene and the ultrafast energy transfer between the O-I interface. Furthermore, those organic counterparts significantly modulate the bindings of charged excitons in monolayer MoSe2 via their precise dielectric environment engineering. The results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using atomically thin O-I heterostructures.

Highly Enhanced Many-Body Interactions in Anisotropic 2D Semiconductors.

Atomically thin two-dimensional (2D) semiconductors have presented a plethora of opportunities for future optoelectronic devices and photonics applications, made possible by the strong light matter interactions at the 2D quantum limit. Many body interactions between fundamental particles in 2D semiconductors are strongly enhanced compared with those in bulk semiconductors because of the reduced dimensionality and, thus, reduced dielectric screening. These enhanced many body interactions lead to the formation of robust quasi-particles, such as excitons, trions, and biexcitons, which are extremely important for the optoelectronics device applications of 2D semiconductors, such as light emitting diodes, lasers, and optical modulators, etc. Recently, the emerging anisotropic 2D semiconductors, such as black phosphorus (termed as phosphorene) and phosphorene-like 2D materials, such as ReSe2, 2D-perovskites, SnS, etc., show strong anisotropic optical and electrical properties, which are different from conventional isotropic 2D semiconductors, such as transition metal dichalcogenide (TMD) monolayers. This anisotropy leads to the formation of quasi-one-dimensional (quasi-1D) excitons and trions in a 2D system, which results in even stronger many body interactions in anisotropic 2D materials, arising from the further reduced dimensionality of the quasi-particles and thus reduced dielectric screening. Many body interactions have been heavily investigated in TMD monolayers in past years, but not in anisotropic 2D materials yet. The quasi-particles in anisotropic 2D materials have fractional dimensionality which makes them perfect candidates to serve as a platform to study fundamental particle interactions in fractional dimensional space. In this Account, we present our recent progress related to 2D phosphorene, a 2D system with quasi-1D excitons and trions. Phosphorene, because of its unique anisotropic properties, provides a unique 2D platform for investigating the dynamics of excitons, trions, and biexcitons in reduced dimensions and fundamental many body interactions. We begin by explaining the fundamental reasons for the highly enhanced interactions in the 2D systems influenced by dielectric screening, resulting in high binding energies of excitons and trions, which are supported by theoretical calculations and experimental observations. Phosphorene has shown much higher binding energies of excitons and trions than TMD monolayers, which allows robust quasi-particles in anisotropic materials at room temperature. We also discuss the role of extrinsic defects induced in phosphorene, resulting in localized excitonic emissions in the near-infrared range, making it suitable for optical telecommunication applications. Finally, we present our vision of the exciting device applications based on the highly enhanced many body interactions in phosphorene, including exciton-polariton devices, polariton lasers, single-photon emitters, and tunable light emitting diodes (LEDs).

Ultrahigh mobility and efficient charge injection in monolayer organic thin-film transistors on boron nitride.

Organic thin-film transistors (OTFTs) with high mobility and low contact resistance have been actively pursued as building blocks for low-cost organic electronics. In conventional solution-processed or vacuum-deposited OTFTs, due to interfacial defects and traps, the organic film has to reach a certain thickness for efficient charge transport. Using an ultimate monolayer of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) molecules as an OTFT channel, we demonstrate remarkable electrical characteristics, including intrinsic hole mobility over 30 cm2/Vs, Ohmic contact with 100 Ω · cm resistance, and band-like transport down to 150 K. Compared to conventional OTFTs, the main advantage of a monolayer channel is the direct, nondisruptive contact between the charge transport layer and metal leads, a feature that is vital for achieving low contact resistance and current saturation voltage. On the other hand, bilayer and thicker C8-BTBT OTFTs exhibit strong Schottky contact and much higher contact resistance but can be improved by inserting a doped graphene buffer layer. Our results suggest that highly crystalline molecular monolayers are promising form factors to build high-performance OTFTs and investigate device physics. They also allow us to precisely model how the molecular packing changes the transport and contact properties.

Epitaxial Ultrathin Organic Crystals on Graphene for High-Efficiency Phototransistors.

Epitaxially grown ultrathin organic semiconductors on graphene show great promise as highly efficient phototransistors. The devices exhibit a strong photoresponse down to the limit of a monolayer organic crystal, with a photoresponsivity higher than 10(4) A W(-1) and a photoconductive gain over 10(8) . The excellent performance is attributed to the high quality of the organic crystal and interface, a unique feature of van der Waals epitaxy.