Solution-processable Transition Metal Oxides And (Sulfur) Iodine Compounds For Photodetectors

Photodetectors can convert optical signals into electrical signals,which plays an important role in imaging technology,environmental monitoring and optical communication.The commercially used photodetectors are mostly designed based on crystalline Si,Si-Ge heterojunction or III-V group semiconductor alloy bulk materials.However,normally these kinds of materials are fragile,expensive etc.,and sometimes demand complecated fabrication processes.Thus it is difficult to meet the needs of next-generation optoelectronic devices which require low power consumption,lightweight design,easy portability,excellent mechanical flexibility,scalability,and low manufacturing cost.Low-dimensional semiconductor materials,such as halide perovskites,inorganic nanocrystals,organic semiconductors,2D materials,etc.,demonstrate unique physical and electronic properties,so low-dimensional materials based wide-band photodetectors with fast response,high sensitivity and low power consumption are expected to become vital components in communications,remote sensing,monitoring and imaging technologies.Therefore,it has attracted a lot of research attention at home and abroad and has become one of the hot issues in the field of information and other fields.Currently widely studied low-dimensional material photodetectors range from deep ultraviolet to near-infrared,and the device's single metric is excellent.However,evaluating device quality cannot be based solely on a single metric,but also needs to consider the balance between various technical metrics.In addition,there are challenges for developing cheap,simple material preparation processes and construct large-area photodetectors based on low-dimensional materials.Therefore,research in this field has important scientific and applied value.Solution-processable has the characteristics of low cost and large scale.The solution-based synthesis usually involves hydrothermal/solvothermal reaction,solution-based self-assembly,template-assisted growth and subsequent heat-treatment processes,etc.Benefitting from low-cost and simple manufacturing process,various solution-processable semiconductor materials have been explored for constructing photodetectors.Therefore,in the view of both fundamental researches and technological applications,we developed a simple solution method,successfully prepared some low-dimensional materials based on transition metal oxides and?sulfur?iodine compounds,and constructed photodetectors based on such materials.We systematically studied their performance,and explored the balance of various technical parameters of the photodetectors in this dissertation.We hereby present some results with respect to the constraction of versatile photodetectors,combing with the exploration of novel materials and device architectures.Specifically,based on the synthesis,structure and dimension of materials,as well as detection mechanisms and spectral response bands,we show a series of recent work on new types of optoelectronic devices.It mainly includes self-powered,low-dimensional,flexible,mixed-dimensionally van der Waals heterostructural,and anomalously negatively photoconductive photodetectors.Some work was on the synthesis of low-dimensional materials at large scale,functional application of optoelectronic device,and multi-field system integration.The main research contents and results are as follows.?.Self-powered UV-Visible Photodetector with Fast Response and High Photosensitivity Employing Fe:TiO₂/n-Si Heterojunction.In this chapter,we reported a facile developing solution process.Two sols of tetrabutyl titanate(C₁₆H₃₆O₄Ti)ethanol solution and anhydrous ferric chloride?FeCl₃?ethanol solution were mixed with magnetic stirring until a uniform light yellow sol was formed.Light yellow sol was spin-coated at Si wafer.Subsequently annealed and spontaneously cooled to room temperature.Namely,Fe:TiO2/n-Si heterojunction nanostructure samples were obtained.Thus,a Fe:TiO₂/n-Si ultraviolet-visible photodetector with fast response,high photosensitivity and self-powered was constructed.As-fabricated devices exhibited excellent photoresponse properties,including high responsivity of 46 mA/W?350 nm?and 60 mA/W?600 nm?with a 0.5 mW?cm^-2 light irradiation under zero bias,as well as an ultrasensitive?on/off ratio up to 10³?,fast?rise/decay time of<10/15 ms?,and broad-band?UV-visible?photodetection with no or low external energy supply.Furthermore,the quantum efficiency of the heterojunction rose up beyond 100%within a broad wavelength range at a small reverse bias of-0.5 V.The self-powered properties originated from the existence of built-in electric field between Fe:TiO₂ and Si,which was confirmed by capacitance-voltage?C-V?measurement,and this field helped to facilitate the separation of photogenerated electron-hole pairs and regulate the electron transport.Moreever,the constructed heterojunction devices exhibited stable,repeatable optoelectronic properties.Such Fe:TiO₂/n-Si heterojunction photodetector might be used for the fast detection of weak-signal at UV-visible range.?.Millimeter-Sized PbI₂ Flakes and Pb₅S₂I₆ Nanowires for Flexible Photodetectors.In this chapter,we developed hydrothermal methods.Lead chloride?PbCl₂?,iodine?I₂?,thiourea??NH₂?₂CS?and distilled water were added to a Teflon-lined autoclave.The autoclave was then maintained at 150°C for 10 hours and subsequently cooled to room temperature naturally.Large-size layered-crystalline Lead?II?Iodide?PbI₂?flakes with sizes up to 5 mm were synthesized.Analysis of growth kinetics revealed that the presence of thiourea and temperature drop promoted the growth of large-scale flakes.Photocurrent measurements of PbI₂ flake photodetector?PD?constructed on flexible polyimide?PI?substrates exhibited a responsivity at 510 nm up to 5 mA·W^-1.Additionally,the device displayed a fast response speed of less than 30 ms.Dark currents are lower than 10 pA at a bias of 5 V.Spectral selectivity of our PbI₂ flakes may potentially be used to narrowband photodetection.The process can also be converted into Pb₅S₂I₆ nanowires?NWs?by changing the ratio of reactants with a broadened spectral response range.In addition,both PbI₂ flake and Pb₅S₂I₆ nanowires flexible photodetectors exhibited excellent flexibility,mechanical stability,folding resistance and long-term stability.Our results on the freestanding growth of large area PbI₂ flakes and Pb₅S₂I₆ NWs may lead to facile flexible device fabrication possibilities.These findings can broaden our fundamental knowledge of van der Waals layered halide semiconductors.Meanwhile,this hydrothermal synthetic approach is simple,scalable,and applicable to other 2D layered materials.?.SbSI Whisker/PbI₂ Flake Mixed-Dimensional van der Waals Heterostructure for Photodetection.Based on the above work on PbI₂,in this chapter,we reported for the first time a large mixed-dimensional vdW heterostructure formed from individual SbSI whisker and individual PbI₂ flake for photodetection.The effective area of the heterojunction reached 10⁵?m²,which is much larger than the junction area of previously reported work.The SbSI/PbI₂ heterostructure showed the van der Waals interaction at the interface verifying the interlayer coupling by Raman spectroscopy.The I_ds-V_ds curve of the device under no illumination has similar rectification characteristics,which proved the formation of SbSI/PbI₂ van der Waals heterojunction.Benefiting from millimeter-sized whisker and flake,we took advantages of the flexibility of operation and vdW junction interface between SbSI and PbI₂.This mixed-dimensional heterostructure is used as an optoelectronics platform due to its larger light-harvesting cross section relative to that of individual material.2D material acted as a current collector while 1D material acted as a current channel.Comparing with device of individual SbSI whisker,vdW heterojunction devices exhibited lower dark currents and broader spectral response range.SbSI/PbI₂ photodetector exhibited a responsivity up to 26.3 mA·W^-1 and showed fast response speed of¹2 ms.In addition,multiple SbSI/PbI₂ photodetectors all exhibited reliable and stable photoelectric characteristics.Large-area growth and scalable approaches lead to efficient transfer of two-dimensional materials and heterostructure assembly.?.The self-driven photoresponse,anomalous negative photoconductivity and resistance switching phenomenon in the B-phase VO₂ nanorods were investigated for the first time.In this chapter,The VO₂?B?nanorods were obtained by developinghydrothermal method.Vanadium oxide?V₂O₅?,oxalic acid?H₂C₂O₄?and ultrapure water were maintained at 200°C for 10 hours and then VO₂?B?nanorods film devices of photodetector and resistive switching were constracted.We found for the first time that near zero bias,unlike in many other semiconductors,the photocurrent generation in VO₂ NRs might be influenced by the photothermoelectric effect and not only by the separation of photoexcited electron-hole pairs.The device demonstrated excellent response in the wide spectral range of 250-850 nm at zero bias during photoelectric measurement.Photoexcitation suppressed the conductivity away from zero bias,observed by wavelength-dependent photoconductance measurements.Resistive switching phenomenon attributed to the surface states of materials by I-V sweeps was also found.The temperature-controlled resistive switching device was constructed by combining with the phase transition material of M-phase VO₂ film.When it reached the phase transition temperature,the device operated with resistive switching and could be used for storage.These intriguing properties of the VO₂?B?NRs pave the way to new applications of this material such as broadband photodetection and the charge storage with low power consumption.Finally,based on the above work,BiSI/SbSI whisker material were synthesized via hydrothermally added bismuth chloride?BiCl₃?/bismuth chloride?SbCl₃?,thiourea??NH₂?₂CS?,iodine?I₂?into aqueous solution and maintained at 180-200°C for 12 hours.Resistive switching and piezoelectric phenomena were found in two-terminal devices constructed of such ferroelectric materials.We give some propositions on material synthesis,device construction,precise measurement,theoretical calculation,and imaging exploration and we also make some prospects.