VO2 /MoO3 Heterojunctions Artificial Optoelectronic Synapse Devices for Near-Infrared Optical Communication.

Artificial optoelectronic synapses (OES) have attracted extensive attention in brain-inspired information processing and neuromorphic computing. However, OES at near-infrared wavelengths have rarely been reported, seriously limiting the application in modern optical communication. Herein, high-performance near-infrared OES devices based on VO2 /MoO3 heterojunctions are presented. The textured MoO3 films are deposited on the sputtered VO2 film by using the glancing-angle deposition technique to form a heterojunction device. Through tuning the oxygen defects in the VO2 film, the fabricated VO2 /MoO3 heterojunction exhibits versatile electrical synaptic functions. Benefiting from the highly efficient light harvesting and the unique interface effect, the photonic synaptic characteristics, mainly including the short/long-term plasticity and learning experience behavior are successfully realized at the O (1342 nm) and C (1550 nm) optical communication wavebands. Moreover, a single OES device can output messages accurately by converting light signals of the Morse code to distinct synaptic currents. More importantly, a 3 × 3 artificial OES array is constructed to demonstrate the impressive image perceiving and learning capabilities. This work not only indicates the feasibility of defect and interface engineering in modulating the synaptic plasticity of OES devices, but also provides effective strategies to develop advanced artificial neuromorphic visual systems for next-generation optical communication systems.

A high-sensitivity SnSe/Si heterojunction position-sensitive detector for ultra-low power detection.

Position-sensitive detectors (PSDs) based on the lateral photovoltaic effect are crucial components in non-contact distance measurement, process control, guidance systems, and other related applications. However, PSDs are challenging due to the narrow spectral range and low sensitivity, limiting further practical application. Here, we present an ultra-sensitive SnSe/Si PSD device. A large-area uniform SnSe nanorod (NR) array film was grown on Si using a glancing-angle magnetron sputtering deposition technique and a SnSe/Si heterojunction PSD device was fabricated. PSDs exhibit an excellent photoresponse in a wide spectral range of 405-980 nm, showing an ultrahigh position sensitivity of 1517.4 mV mm-1 and an excellent spectral sensitivity of 4 × 104 V W-1. More importantly, the detection limit power of the device is as low as 10 nW, indicating the outstanding potential for weak light detection. Based on the unique structural features and interface effect, the mechanisms for the remarkable performance of the fabricated SnSe/Si PSD device are clarified. This work indicates the large potential of SnSe/Si heterojunctions as a promising material for ultrasensitive optical position-sensitive devices.

High-Performance Flexible Broadband Photothermoelectric Photodetectors Based on Tellurium Films.

Mid- and far-infrared photodetectors that can operate at room temperature are essential for both civil and military applications. However, the widespread use of mid-to-far-infrared photonic technology faces challenges due to the need for low-temperature cooling of existing commercial semiconductors and the limited optical absorption efficiency of two-dimensional materials. We have utilized the photothermoelectric effect to fabricate a self-powered, broadband, and high-performance photodetector based on a one-dimensional tellurium nanorod array film. The device surpasses energy band gap limitations, functioning even at wavelengths up to approximately 10,600 nm. In particular, the detectivity of the device can reach 4.8 × 109 Jones at 4060 nm under room-temperature conditions, which is an order of magnitude higher than that of commercially available photodetectors. It demonstrates fast response and recovery times of 8.3 and 8.8 ms. Furthermore, the device demonstrates outstanding flexibility withstanding over 300 bending cycles and environmental stability. These results suggest a viable approach for designing and developing high-performance, room-temperature, wearable optoelectronic devices.

All-in-One Optoelectronic Logic Gates Enabled by Bipolar Spectral Photoresponse of CdTe/SnSe Heterojunction.

Optoelectronic logic gate devices (OLGDs) have attracted significant attention in high-density information processors; however, multifunctional logic operation in a single device is technically challenging due to the unidirectional electrical transport. In this work, we deliberately design all-in-one OLGDs based on self-powered CdTe/SnSe heterojunction photodetectors. The SnSe nanorod (NR) array is grown on the sputtered CdTe film via a glancing-angle deposition technique to form a heterojunction device. At the interface, the photovoltaic (PV) effect in the CdTe/SnSe heterojunction and the photothermoelectric (PTE) effect from the SnSe NRs are combined together to induce the reversed photocurrent, leading to a unique bipolar spectral response. The competition between PV and PTE in different spectral ranges is thus employed to control the photocurrent polarity, and five basic logic gates of OR, AND, NAND, NOR, and NOT can be performed just with a single heterojunction. Our findings indicate the large potentials of the CdTe/SnSe heterojunctions as logic units in next-generation sensing-computing systems.

Si/SnSe-Nanorod Heterojunction with Ultrafast Infrared Detection Enabled by Manipulating Photo-Induced Thermoelectric Behavior.

Photothermal detectors have attracted tremendous research interest in uncooled infrared imaging technology but with a relatively slow response. Here, Si/SnSe-nanorod (Si/SnSe-NR) heterojunctions are fabricated as a photothermal detector to realize high-performance infrared response beyond the bandgap limitation. Vertically standing SnSe-NR arrays are deposited on Si by a sputtering method. Through manipulating the photoinduced thermoelectric (PTE) behavior along the c-axis, the Si/SnSe-NRs heterojunction exhibits a unique four-stage photoresponse with a high photoresponsivity of 106.3 V W-1 and high optical detectivity of 1.9 × 1010 cm Hz1/2 W-1 under 1342 nm illumination. Importantly, an ultrafast infrared photothermal response is achieved with the rise/fall time of 11.3/258.7 µs. Moreover, the coupling effect between the PTE behavior and external thermal excitation enables an improved response by 288.4%. The work not only offers a new strategy to develop high-speed photothermal detectors but also performs a deep understanding of the PTE behavior in a heterojunction system.

Photothermoelectric SnTe Photodetector with Broad Spectral Response and High On/Off Ratio.

A broadband photodetector with high performance is highly desirable for the optoelectric and sensing application. Herein, we report a "photo-thermo-electric" (PTE) detector based on an ultrathin SnTe film. The (001)-oriented SnTe films with the wafer size scale are epitaxially grown on the surface of sodium chloride crystals by a scalable sputtering method. Due to the giant PTE effect under laser spot excitation on the asymmetric position between two terminals, a built-in electrical field is produced to drive bulk carriers for a self-powered photodetector, leading to a broad spectral response in the wavelength range from 404 nm to 10.6 µm far beyond the limitation of the energy band gap. Significantly, the photodetector displays a high on/off photoswitching ratio of over 105 with a suppressed dark current, which is 4-5 orders of magnitude higher than that of other reported SnTe-based detectors. Under zero external bias, the device yields the highest detectivity of ∼1.3 × 1010 cm Hz1/2 W-1 with a corresponding responsivity of ∼3.9 mA W-1 and short rising/falling times of ∼78/84 ms. Furthermore, the photodetector transferred onto the flexible template exhibits excellent mechanical flexibility over 300 bending cycles. These findings offer feasible strategies toward designing and developing low-power-consumption wearable optoelectronics with competitive performance.

Flexible SnSe Photodetectors with Ultrabroad Spectral Response up to 10.6 µm Enabled by Photobolometric Effect.

A broad spectral response is highly desirable for radiation detection in modern optoelectronics; however, it still remains a great challenge. Herein, we report a novel ultrabroadband photodetector based on a high-quality tin monoselenide (SnSe) thin film, which is even capable of detecting photons with energies far below its optical band gap. The wafer-size SnSe ultrathin films are epitaxially grown on sodium chloride via the 45° in-plane rotation by employing a sputtering method. The photodetector delivers sensitive detection to ultraviolet-visible-near infrared (UV-Vis-NIR) lights in the photoconductive mode and shows an anomalous response to long-wavelength infrared at room temperature. Under the mid-infrared light of 10.6 µm, the fabricated photodetector exhibits a large photoresponsivity of 0.16 A W-1 with a fast response rate, which is ∼3 orders of magnitude higher than other results. The thermally induced carriers from the photobolometric effect are responsible for the sub-bandgap response. This mechanism is confirmed by a temperature coefficient of resistance of -2.3 to 4.4% K-1 in the film, which is comparable to that of the commercial bolometric detectors. Additionally, the flexible device transferred onto polymer templates further displays high mechanical durability and stability over 200 bending cycles, indicating great potential toward developing wearable optoelectronic devices.

Wafer-size growth of 2D layered SnSe films for UV-Visible-NIR photodetector arrays with high responsitivity.

Due to its excellent electrical and optical properties, tin selenide (SnSe), a typical candidate of two-dimensional (2D) semiconductors, has attracted great attention in the field of novel optoelectronics. However, the large-area growth of high-quality SnSe films still remains a great challenge, which limits their practical applications. Here, wafer-size SnSe ultrathin films with high uniformity and crystallization were deposited via a scalable magnetron sputtering method. The results showed that the SnSe photodetector was highly sensitive to a broad range of wavelengths in the UV-visible-NIR range, especially showing an extremely high responsivity of 277.3 A W-1 with the corresponding external quantum efficiency of 8.5 × 104% and detectivity of 7.6 × 1011 Jones. These figures of merits are among the best performances for the sputter-fabricated 2D photodetector devices. The photodetecting mechanisms based on a photogating effect induced by the trapping effect of localized defects are discussed in detail. The results indicate that the few-layered SnSe films obtained from sputtering growth have great potential in the design of high-performance photodetector arrays.

Theoretical study of T shaped phenothiazine/carbazole based organic dyes with naphthalimide as π-spacer for DSSCs.

Eight novel T shaped phenothiazine/carbazole based organic dyes with naphthalimide as π-spacer were designed, and the geometries, electronic structures, and optical features of these isolated dyes and dye-(TiO2)9 systems were investigated with density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations. Some quantify factors influencing the energy conversion efficiency (PCE) such as the light harvesting efficiency (LHE), electron injection driving force (ΔGinject) and dye regeneration driving force (ΔGreg) were also calculated for dye-sensitized solar cells (DSSCs) applications. It is found that these dyes show a good performance of electron injection and dye regeneration owing to the proper value of ΔGinject and ΔGreg. The optimized geometries of the non-planar molecular configuration of donor and the planar structure of the naphthalimide conjugated bridge are beneficial to efficient intramolecular charge transfer and the suppression of molecular aggregation. The properties about the electronic structure and absorption spectra indicate that replacement of benzene with thiophene unit near to cyanoacetic acid acceptor can generate more efficient conjugation effect and achieve red shift of absorption spectra, resulting a higher Jsc and Voc in DSSCs device. The theoretical results reveal that DTPH2, DTPH4, DTCA2 and DTCA4 would be used as potential sensitizers for DSSCs applications.

DFT/TD-DFT study of novel T shaped phenothiazine-based organic dyes for dye-sensitized solar cells applications.

Five novel T shaped phenothiazine-based organic dyes DTTP1~5 with different spacers at N (10) position were designed. The geometries, electronic structures, absorption spectra, electron transfer and injection properties of these isolated dyes and dye/(TiO2)9 systems were investigated via density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculation. The optimized geometries indicate that these T shaped dyes show non-planar conformations, which are helpful in suppressing the close intermolecular π-π aggregation in device and enhancing thermal stability. The calculated results indicate that type of π-conjugated spacers can affect the molecular absorption spectra. Introduction of thiophene-benzothiadizole-thiophene unit as π-conjugated spacer can most effectively shift the light absorption to near infrared region and enhance the light harvesting efficiency (LHE). Moreover, it is found that these dyes show a good performance of electron injection and dye regeneration owing to the proper electron injection driving force (ΔGinject) and dye regeneration driving force (ΔGreg). The theoretical results reveal that these dyes could be used as potential sensitizers for DSSCs, and DTTP4 would be the most plausible sensitizer for high-efficiency DSSCs due to the narrow HOMO-LUMO energy gap (ΔH-L), broad absorption spectrum, high LHE value, and large dipole moment (µnormal).

Highly Enhanced H2 Sensing Performance of Few-Layer MoS2/SiO2/Si Heterojunctions by Surface Decoration of Pd Nanoparticles.

A novel few-layer MoS2/SiO2/Si heterojunction is fabricated via DC magnetron sputtering technique, and Pd nanoparticles are further synthesized on the device surface. The results demonstrate that the fabricated sensor exhibits highly enhanced responses to H2 at room temperature due to the decoration of Pd nanoparticles. For example, the Pd-decorated MoS2/SiO2/Si heterojunction shows an excellent response of 9.2 × 103% to H2, which is much higher than the values for the Pd/SiO2/Si and MoS2/SiO2/Si heterojunctions. In addition, the H2 sensing properties of the fabricated heterojunction are dependent largely on the thickness of the Pd-nanoparticle layer and there is an optimized Pd thickness for the device to achieve the best sensing characteristics. Based on the microstructure characterization and electrical measurements, the sensing mechanisms of the Pd-decorated MoS2/SiO2/Si heterojunction are proposed. These results indicate that the Pd decoration of few-layer MoS2/SiO2/Si heterojunctions presents an effective strategy for the scalable fabrication of high-performance H2 sensors.

Large Lateral Photovoltaic Effect in MoS2/GaAs Heterojunction.

Molybdenum disulfide (MoS2) nanoscaled films are deposited on GaAs substrates via magnetron sputtering technique, and MoS2/GaAs heterojunctions are fabricated. The lateral photovoltaic effect (LPE) of the fabricated MoS2/GaAs heterojunctions is investigated. The results show that a large LPE can be obtained in the MoS2/n-GaAs heterojunction. The LPE exhibits a linear dependence on the position of the laser illumination and the considerably high sensitivity of 416.4 mV mm-1. This sensitivity is much larger than the values in other reported MoS2-based devices. Comparatively, the LPE in the MoS2/p-GaAs heterojunction is much weaker. The mechanisms to the LPE are unveiled by constructing the energy-band alignment of the MoS2/GaAs heterojunctions. The excellent LPE characteristics make MoS2 films combined with GaAs semiconductors promising candidates for the application of high-performance position-sensitive detectors.