vdWs Heterostructures Research Articles

Intriguing optoelectronic and visible-light activated photocatalytic properties of 2D AlN/GaN and TMDCs (MX2; M = Mo/W, X = S/Se) monolayers and their bilayer vdWs heterostructures

Intriguing optoelectronic and visible-light activated photocatalytic properties of 2D AlN/GaN and TMDCs (MX2; M = Mo/W, X = S/Se) monolayers and their bilayer vdWs heterostructures

Interface‐Induced Anomalous Behavior of Magnetism in FexGeTe2/Pt Bilayer

AbstractInterface engineering is a promising strategy for controlling the Curie temperature (Tc) and perpendicular magnetic anisotropy (PMA) in magnetic 2D van der Waals (2D vdWs)‐based heterostructures. However, establishing high‐quality interface structures in magnetic 2D vdWs/metal stacks, crucial for maximizing interface effects, remains a significant challenge. Here, a Fe5‐xGeTe2/Pt (F5GT/Pt) prototype with a superior interface quality is achieved using a low‐power physical vapor deposition technique. The magnetic properties of the F5GT/Pt heterostructures are strongly influenced by employing the specific physical deposition method. Stable ferromagnetism at 400 K is observed when depositing Pt atoms with relatively high energy, despite the Tc of pristine F5GT being below 300 K. This unexpected high‐temperature ferromagnetism is attributed to the formation of a ferromagnetic alloy at the interface, commonly present in vdWs‐based stacks fabricated through physical deposition but often overlooked. The deposit of Pt atoms with ultralow energy leads to the formation of a unique Fe5‐xGeTe2/Fe3‐xGeTe2 heterojunction at the interface, significantly enhancing the PMA. This work emphasizes the importance of interface structures in vdWs‐based devices, suggesting that controlling the growth process offers an effective approach to construct and engineer vdWs heterostructures, thus improving the performance and introducing new functionalities to spintronic devices.

High performance self-driven broadband photodetector for polarized imaging based on novel ZrS3/ReSe2 van der Waals heterojunction

The distinctive characteristics of anisotropic two-dimensional (2D) materials, including in-plane anisotropy of optical absorption and carrier mobility, render them exceptionally suitable for application in the field of polarization detection and as a novel platform for the polarization imaging. Meanwhile, the consolidation of diverse functionalities within a single photodetector is highly anticipated to meet the demands of some special scenarios. Herein, a novel ZrS3/ReSe2 van der Waals (vdWs) heterostructure device was successfully constructed to realize polarization-sensitive, self-powered, and broadband photodetection and imaging. Owing to the built-in electric field of the type-II band alignment within the heterojunction, the device achieves a self-powered photoresponse ranging from 300 to 980 nm, an ultralow dark currentt ∼1 pA, and a commendable rise/decay time of 0.35/0.28 ms. Additionally, it has been demonstrated that the self-driven photodetector possesses a polarization-sensitivity with a notable anisotropic ratio about 2.02 (1.98) under 490 nm (980 nm) light illumination with zero bias, coupled with an excellent repeatability and stability. Furthermore, we also demonstrate the polarization imaging capabilities of the device in visible and near-infrared spectrum, realizing a contrast-enhanced degree of linear polarization imaging. This work paves a new platform to develop heterojunction photodetectors for high performance polarization-sensitive photodetection and next-generation polarized imaging.

Graphene electrode-enhanced InSe/WSe2 van der Waals heterostructure for high-performance broadband photodetector with imaging capabilities

2D vdWs heterostructure is realized as a powerful technique for tuning the optoelectronic properties and thus developing the future generation optoelectronic devices, especially for photodetectors. However, photodetectors based on 2D vdWs heterostructure suffer from responsivity, detectivity, and large photoresponse speed inhibits their applications in further diverse areas. Here, a graphene electrode-based InSe/WSe2 vdWs heterostructure is proposed aiming to develop a high-performance photodetector. Owing to the graphene electrode, the heterostructure devices build a strong electronic field at the surface of the InSe/WSe2 vdWs heterostructure. As a result, the device shows excellent optoelectronic characteristics such as broad band photoresponse ranging from 532 nm (visible) to 1100 nm (near infrared) including a high responsivity of 829.7 AW−1, a detectivity of 2.81×1014 Jones and a rapid photoresponse of 10 µs. Significantly, as presented photodetector device is applied in an imaging system, showcasing its capability for high-contrast photodetection. These findings highlight that the potential of graphene electrode integration in 2D vdWs heterostructure provides an effective roadmap for developing the advancing next generation of optoelectronic devices.

The interlayer twist effectively regulates interlayer excitons in InSe/Sb van der Waals heterostructure

The interlayer twist angle endows a new degree of freedom to manipulate the spatially separated interlayer excitons in van der Waals (vdWs) heterostructures. Herein, we find that the band-edge Γ-Γ interlayer excitation directly forms interlayer exciton in InSe/Sb heterostructure, different from that of transition metal dichalcogenides (TMDs) heterostructures in two-step processes by intralayer excitation and transfer. By tuning the interlayer coupling and breathing vibrational modes associated with the Γ-Γ photoexcitation, the interlayer twist can significantly adjust the excitation peak position and lifetime of recombination. The interlayer excitation peak in InSe/Sb heterostructure can shift ~400 meV, and the interlayer exciton lifetime varies in hundreds of nanoseconds as a periodic function of the twist angle (0°–60°). This work enriches the understanding of interlayer exciton formation and facilitates the artificial excitonic engineering of vdWs heterostructures.

Interlayer Charge Transition and Broadband Polarization Photodetection and Imaging Based on In2Se3/ReS2 van der Waals Heterostructure

Abstract2D van der Waal (vdWs) heterostructures present unique optoelectronic characteristics, making them favorable layer structures for constructing promising optoelectronic devices with multifunctional applications. Nevertheless, as a result of significant interface recombination of the photogenerated electron‐hole pairs and the presence of various absorption edges within constituent layers, they are prone to experiencing low carrier collection efficiency. In this work, a combined theoretical and experimental investigation are presented on the In2Se3/ReS2 vdWs heterostructure, aimed at developing high‐performance and broadband photodetector with multifunctionalities. In theoretical investigations, it is observed that, by adjusting the polarization states (+P to −P) in the In2Se3 layer, band alignment can be effectively tuned from type‐I to type‐II, providing a narrow bandgap of ≈0.65 eV, which is beyond that of their individual constituents. As a photodetector, the device shows broadband photoresponse ranging from 532 to 1550 nm with ultrahigh responsivity (99.36 AW−1), detectivity (3.5 × 1013 Jones), and external quantum efficiency (34195%). Additionally, competitive polarization sensitivity across the broad spectrum and imagining capability are observed with In2Se3/ReS2 vdWs heterostructure. This study demonstrates that In2Se3/ReS2 vdWs heterostructure device provides a promising technique for developing high‐performance 2D optoelectronic devices with multifunctionalities.

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.

Mixed-Dimensional Assembly Strategy to Construct Reduced Graphene Oxide/Carbon Foams Heterostructures for Microwave Absorption, Anti-Corrosion and Thermal Insulation

HighlightsReduced graphene oxide/carbon foams (RGO/CFs) vdWs heterostructures are efficiently fabricated via a simple mixed-dimensional assembly strategy.Linkage effect of optimized impedance matching and enhanced dielectric loss abilities endows the excellent microwave absorption performances of RGO/CFs vdWs heterostructures.Multiple functions such as good corrosion resistance performances and outstanding thermal insulation capabilities can be integrated into RGO/CFs vdWs heterostructures.

Design of van der Waals heterostructures composed of g-C3N4 and III-V materials for solar cell

As society develops, people pay more attention to the application of solar cell to harvest inexhaustible solar power, which has a benefit of greenness and cleanness. In this paper, we construct 14 heterostructures composed of g-C3N4 and Ⅲ-Ⅴ materials based on first-principles calculations. We systematically analyse their stability property and find a periodic rule of binding energy change about the atomic number of Ⅲ-Ⅴ material. We select 9 stable van der Waals (vdWs) heterostructures and further analyse their band alignment and power conversion efficiency (PCE). We find GaAs-D is a type-Ⅱ heterostructure with indirect band gap of 1.38 eV and the PCE reaches 15.2%. The construction of GaAs-D effectively promotes the optical absorption in visible-spectrum compared with two monolayers. This paper systematically analyses the stability of constructed vdWs heterostructures composed of g-C3N4 and Ⅲ-Ⅴ materials, electronic band structures and their potential application as solar cell.

Field-free spin–orbit torque switching of magnetic tunnel junction structure based on two-dimensional van der Waals WTe2

In recent years, two-dimensional van der Waals (2D vdWs) heterostructures have attracted great research interest due to their great potential in fundamental physics research and spintronic devices (such as MTJs). Due to its excellent scalability, controllable magnetism and out-of-plane anisotropy, the compact nonvolatile memory controller (NV-MC) based on spintronics is expected to solve the memory bottle-neck problem. At present, a series of in-depth studies have been conducted on advanced 2D vdWs materials, such as MoS2, WSe2, and Fe3GeTe2 (FGT). The results show that the 2D vdWs materials have great TMR value and high SOT switching efficiency, both theoretically reported and experimentally verified. In the paper, a novel MTJ device based on the FGT/WTe2 heterostructure is proposed. In the absence of an external magnetic field, the magnetization direction of the MTJ free layer can still be reversed with certainty when the unipolar write current reaches about 5 mA. Moreover, the DMI effect generated between 2-D material/FM interfaces is also considered, which can promote the performance of SOT-MTJ without the external field. The reading reliability of SOT-MRAM is improved in comparison with the traditional CoFeB-based MTJ device.

Broadband nonlinear optical response and sub-picosecond carrier dynamics in graphene-SnSe2 van der Waals heterostructures.

The van der Waals (vdWs) heterostructures, with vertical layer stacking structure of various two-dimensional (2D) materials, maintain the reliable photonic characteristics while compensating the shortcomings of the participating individual components. In this work, we combine the less-studied multilayer tin selenide (SnSe2) thin film with one of the traditional 2D materials, graphene, to fabricate the graphene-based vdWs optical switching element (Gr-SnSe2) with superior broadband nonlinear optical response. The transient absorption spectroscopy (TAS) measurement results verify that graphene acts as the recombination channel for the photogenerated carrier in the Gr-SnSe2 sample, and the fast recovery time can be reduced to hundreds of femtoseconds which is beneficial for the optical modulation process. The optical switching properties are characterized by the I-scan measurements, exhibiting a saturable energy intensity of 2.82 mJ·cm-2 (0.425 µJ·cm-2) and a modulation depth of 15.6% (22.5%) at the wavelength of 1030 nm (1980nm). Through integrating Gr-SnSe2 with a cladding waveguide, high-performance picosecond Q-switched operation in the near-infrared (NIR) and mid-infrared (MIR) spectral regions are both achieved. This work experimentally demonstrates the great potential of graphene-based vdWs heterostructures for applications in broadband ultrafast photonics.

Stacking engineering induced Z-scheme MoSSe/WSSe heterostructure for photocatalytic water splitting.

Stacking engineering is a popular method to tune the performance of two-dimensional materials for advanced applications. In this work, Jansu MoSSe and WSSe monolayers are constructed as a van der Waals (vdWs) heterostructure by different stacking configurations. Using first-principle calculations, all the relaxed stacking configurations of the MoSSe/WSSe heterostructure present semiconductor properties while the direct type-II band structure can be obtained. Importantly, the Z-scheme charge transfer mode also can be addressed by band alignment, which shows the MoSSe/WSSe heterostructure is an efficient potential photocatalyst for water splitting. In addition, the built-in electric field of the MoSSe/WSSe vdWs heterostructure can be enhanced by the S-Se interface due to further asymmetric structures, which also results in considerable charge transfer comparing with the MoSSe/WSSe vdWs heterostructure built by the S-S interface. Furthermore, the excellent optical performances of the MoSSe/WSSe heterostructure with different stacking configurations are obtained. Our results provide a theoretical guidance for the design and control of the two-dimensional heterostructure as photocatalysts through structural stacking.

Catalytic synergy of WS2-anchored PdSe2 for highly sensitive hydrogen gas sensor.

Hydrogen (H2) is widely used in industrial processes and is one of the well-known choices for storage of renewable energy. H2 detection has become crucial for safety in manufacturing, storage, and transportation due to its strong explosivity. To overcome the issue of explosion, there is a need for highly selective and sensitive H2 sensors that can function at low temperatures. In this research, we have adequately fabricated an unreported van der Waals (vdWs) PdSe2/WS2 heterostructure, which exhibits exceptional properties as a H2 sensor. The formation of these heterostructure devices involves the direct selenization process using chemical vapor deposition (CVD) of Pd films that have been deposited on the substrate of SiO2/Si by DC sputtering, followed by drop casting of WS2 nanoparticles prepared by a hydrothermal method onto device substrates including pre-patterned electrodes. The confirmation of the heterostructure has been done through the utilization of powder X-ray diffraction (XRD), depth-dependent X-ray photoelectron spectroscopy (XPS) and field-emission scanning electron microscopy (FE-SEM) techniques. Also, the average roughness of thin films is decided by Atomic Force Microscopy (AFM). The comprehensive research shows that the PdSe2/WS2 heterostructure-based sensor produces a response that is equivalent to 67.4% towards 50 ppm H2 at 100 °C. The response could be a result of the heterostructure effect and the superior selectivity for H2 gas in contrast to other gases, including NO2, CH4, CO and CO2, suggesting tremendous potential for H2 detection. Significantly, the sensor exhibits fast response and a recovery time of 31.5 s and 136.6 s, respectively. Moreover, the explanation of the improvement in gas sensitivity was suggested by exploiting the energy band positioning of the PdSe2/WS2 heterostructure, along with a detailed study of variations in the surface potential. This study has the potential to provide a road map for the advancement of gas sensors utilizing two-dimensional (2D) vdWs heterostructures, which exhibit superior performance at low temperatures.

Mid-Infrared Bipolar and Unipolar Linear Polarization Detections in Nb2 GeTe4 /MoS2 Heterostructures.

Detecting and distinguishing light polarization states, one of the most basic elements of optical fields, have significant importance in both scientific studies and industry applications. Artificially fabricated structures, e.g., metasurfaces with anisotropic absorptions, have shown the capabilities of detecting polarization light and controlling. However, their operations mainly rely on resonant absorptions based on structural designs that are usually narrow bands. Here, a mid-infrared (MIR) broadband polarization photodetector with high PRs and wavelength-dependent polarities using a 2D anisotropic/isotropic Nb2 GeTe4 /MoS2 van der Waals (vdWs) heterostructure is demonstrated. It is shown that the photodetector exhibits high PRs of 48 and 34 at 4.6 and 11.0µm wavelengths, respectively, and even a negative PR of -3.38 for 3.7µm under the zero bias condition at room temperature. Such interesting results can be attributed to the superimposed effects of a photovoltaic (PV) mechanism in the Nb2 GeTe4 /MoS2 hetero-junction region and a bolometric mechanism in the MoS2 layer. Furthermore, the photodetector demonstrates its effectiveness in bipolar and unipolar polarization encoding communications and polarization imaging enabled by its unique and high PRs.

Kinetics of Nanobubbles in Tiny-Angle Twisted Bilayer Graphene.

Realization of high-quality van der Waals (vdWs) heterostructures by stacking two-dimensional (2D) layers requires atomically clean interfaces. Because of strong adhesion between the constituent layers, the vdWs forces could drive trapped contaminants together into submicron-size "bubbles", which leaves large interfacial areas atomically clean. Here, we study the kinetics of nanobubbles in tiny-angle twisted bilayer graphene (TBG) and our results reveal a substantial influence of the moiré superlattice on the motion of nanoscale interfacial substances. Our experiments indicate that the bubbles will mainly move along the triangular network of domain boundaries in the tiny-angle TBG when the sizes of the bubbles are comparable to that of an AA-stacking region. When the size of the bubble is smaller than that of an AA-stacking region, the bubble becomes motionless and is fixed in the AA-stacking region, because of its large out-of-plane corrugation.

Multiple modulation of magnetism in two-dimensional FeCl2/In2Se3 van der Waals heterostructure

The effective modulation of magnetism for two-dimensional (2D) materials not only has potential applications in nanoscale spintronic devices but also has urgent demands in modern industries. In this work, we report the discovery of ferroelectrically (FE) tunable orbital reconstruction in FeCl2/In2Se3 2D van der Waals (vdWs) heterostructures (HSs), which leads to a transition from ferromagnetic (FM) order to ferrimagnetic order (intra-layer magnetic coupling is AFM3 arrangement). The reversible FE polarization not only enables the easy magnetization axis to be tuned from the out-of-plane to in-plane direction but also reduces the net magnetization strength from 31.87 to −0.18 μB/f.u. Based on the charge density differences and the density of states analysis, the preference of FM and AFM3 arrangement can be reasonably explained by the Goodenough–Kanamori–Anderson rule. The ferroelectric switching enables nonvolatile electric control of magnetic order and anisotropy, offering significant potential for high-efficiency nanodevices and nonvolatile information storage. Moreover, modulation of magnetism (magnetic moment from −0.18 to 0.18 μB/f.u.) can also be achieved through interlayer sliding in the −P case, providing a way to control magnetism in 2D vdWs HSs.

Self-Powered Sb2Te3/MoS2 Heterojunction Broadband Photodetector on Flexible Substrate from Visible to Near Infrared

Van der Waals (vdWs) heterostructures, assembled by stacking of two-dimensional (2D) crystal layers, have emerged as a promising new material system for high-performance optoelectronic applications, such as thin film transistors, photodetectors, and light-emitters. In this study, we showcase an innovative device that leverages strain-tuning capabilities, utilizing a MoS2/Sb2Te3 vdWs p-n heterojunction architecture designed explicitly for photodetection across the visible to near-infrared spectrum. These heterojunction devices provide ultra-low dark currents as small as 4.3 pA, a robust photoresponsivity of 0.12 A W−1, and reasonable response times characterized by rising and falling durations of 0.197 s and 0.138 s, respectively. These novel devices exhibit remarkable tunability under the application of compressive strain up to 0.3%. The introduction of strain at the heterojunction interface influences the bandgap of the materials, resulting in a significant alteration of the heterojunction’s band structure. This subsequently shifts the detector’s optical absorption properties. The proposed strategy of strain-induced engineering of the stacked 2D crystal materials allows the tuning of the electronic and optical properties of the device. Such a technique enables fine-tuning of the optoelectronic performance of vdWs devices, paving the way for tunable high-performance, low-power consumption applications. This development also holds significant potential for applications in wearable sensor technology and flexible electro-optic circuits.

Transport properties of B and N sites vacancy defects graphene/h-BN heterostructures: First-principles study

First-principles calculations based on spin-polarized DFT-D2 approach have been carried out to study the transport properties of graphene/h-BN (G/h-BN) heterostructure (HS), 1 N vacancy defect in G/h-BN (G/h-BN_1 N), nearest neighbour of 1 N & 1B vacancy defects in G/h-BN (G/h-BN_nBN), and alternate zone of 1 N & 1B vacancy defects in G/h-BN (G/h-BN_aBN) materials. Transport properties like electrical conductivity (σ), electronic contribution of thermal conductivity (K), Seebeck coefficient (S) and thermoelectric power factor (P) are estimated through Boltzmann transport equations (BTE) within constant relaxation time approximation (RTA). Quantum ESPRESSO and BoltzTrap packages are used as a computational tool in the calculations. The considered materials are found to be stable van der Waals (vdWs) HS. The σ and K of graphene, h-BN, G/h-BN, G/h-BN_1 N, G/h-BN_nBN and G/h-BN_aBN are found to be increasing monotonically with temperature. The defected materials have slightly less value of σ than that of G/h-BN due to the effect of their effective mass and carrier concentration. At room temperature, K of G/h-BN, G/h-BN_nBN and G/h-BN_aBN retains similar value, while K of G/h-BN_1 N has increased slightly because the increase in Fermi velocity of electrons near Fermi level due to the reduction in effective mass thereby increasing the mean free path. We have analyzed the temperature dependent (constant value of energy) and energy dependent (constant value of temperature) S and P of above-mentioned materials, and found that S and P of defected HS are higher than that of non-defected HS materials due to combine dependence of n, T and m* on the value of S. The estimated S and P values of considered materials are consistent with reported value. Hence, they have budding employment in the thermoelectric devices.

Bi2O2Se Nanowire/MoSe2 Mixed-Dimensional Polarization-Sensitive Photodiode with a Nanoscale Ultrafast-Response Channel.

In recent years, polarization-sensitive photodiodes based on one-dimensional/two-dimensional (1D/2D) van der Waals (vdWs) heterostructures have garnered significant attention due to the high specific surface area, strong orientation degree of 1D structures, and large photo-active area and mechanical flexibility of 2D structures. Therefore, they are applicable in wearable electronics, electrical-driven lasers, image sensing, optical communication, optical switches, etc. Herein, 1D Bi2O2Se nanowires have been successfully synthesized via chemical vapor deposition. Impressively, the strongest Raman vibration modes can be achieved along the short edge (y-axis) of Bi2O2Se nanowires with high crystalline quality, which originate from Se and Bi vacancies. Moreover, the Bi2O2Se/MoSe2 photodiode designed with type-II band alignment demonstrates a high rectification ratio of 103. Intuitively, the photocurrent peaks are mainly distributed in the overlapped region under the self-powered mode and reverse bias, within the wavelength range of 400-nm. The resulting device exhibits excellent optoelectrical performances, including high responsivities (R) and fast response speed of 656 mA/W and 350/380 μs (zero bias) and 17.17 A/W and 100/110 μs (-1 V) under 635 nm illumination, surpassing the majority of reported mixed-dimensional photodiodes. The most significant feature of our photodiode is its highest photocurrent anisotropic ratio of ∼2.2 (-0.8 V) along the long side (x-axis) of Bi2O2Se nanowires under 635 nm illumination. The above results reveal a robust and distinctive correlation between structural defects and polarized orientation for 1D Bi2O2Se nanowires. Furthermore, 1D Bi2O2Se nanowires appear to be a great potential candidate for high-performance rectifiers, polarization-sensitive photodiodes, and phototransistors based on mixed vdWs heterostructures.

Optical anisotropy and polarization selectivity in MoS2/Ta2NiSe5 van der Waals heterostructures

In-plane anisotropy induced by a low-symmetric lattice structure in two-dimensional (2D) van der Waals (vdWs) materials has significantly promoted their applications in optoelectronic devices, especially in polarization photodetection. Given the mature preparation technology of transition metal dichalcogenides (TMDCs), introducing artificial anisotropy into symmetric TMDCs becomes a promising way to trigger more excellent functionalities beyond their intrinsic properties. Herein, monolayer MoS2 in the MoS2/Ta2NiSe5 vdWs heterostructure presents obvious anisotropic optical properties confirmed by polarized Raman and photoluminescence spectra, which is mainly ascribed to the uniaxial strain via strong interlayer couplings. Moreover, the MoS2/Ta2NiSe5 heterojunction endows unique orientation-selected polarized absorbance. In the visible region, the dominant polarized orientation is along the armchair axis while rotated to the perpendicular direction in the near-infrared spectrum, which is mainly ascribed to the intrinsic linear dichroism of Ta2NiSe5. The results suggest that the MoS2/Ta2NiSe5 heterostructure provides a promising platform for artificial regulation of the optoelectronic properties of symmetric MoS2 and integrated optical applications in the polarization-sensitive photodetection.