Optoelectronic Properties Of Novel Graphene-based Heterostructures

Low-dimensional materials,such as carbon nanotubes,graphene,and transition metal dichalcogenides,are of great interest as promising candidates for optoelectronic applications due to their remarkable physical properties.In particular,van der Waals(vdWs)heterostuctures have been successfully fabiricated based on these low dimensional materials recently,which open up a new avenue for the study of their fundamental physical peroperties and the design of functional devices.Improving the properties of heterostructures and understanding their physical mechanisms(including those at the interfaces)are prerequisites for the controllable use of the composite materials.Here,two new types of graphene-based vdWs heterostructures are fabricated,and their optoelectronic properties as well as the charge transfer dynamics,etc.,are systematically investigated.Graphene is an attractive material for photonics and optoelectronics because it offers a broad spectral absorption and a fast response time.However,the photoresponsivity of graphene photodetector(～mA/W)is limited by the low absorption cross-section and ultrafast recombination rate of photoexcited carriers.In the past few years,highly responsive graphene photodetectors have been demonstrated primarily by employing hybrid systems with light harvesters,such as quantum-dots,organic molecules and transition metal dichalcogenides.However,these strategies are at the cost of response speed or with low gains.Here,we demonstrate an all-carbon flexible and transparent photodetector composed of graphene-carbon nanotube heterostrcture for the first time.One-dimensional carbon nanotubes and two-dimensional graphene share the same sp2 hybridization of carbon atoms,and can be merged into hybrid systems with the intimate electronic coupling.Integrating monolayer graphene onto carbon nanotube networks reinforces the mechanical strength and improves the electrical conductivity without deteriorating the optical transparency.Due to the strong light absorption of carbon nanotubes,high mobility of graphene and excellent mechanical properties of the heterostructure,the hybrid device exhibits a balanced overall performance with a high photoresponsivity(～51 A/W)and a fast response time(～40 ms)upon visible irradiation.In addition,the hybrid device shows superior flexibility and good robustness under harsh strain and repeated bending tests,which indicates the potential applications of the all-carbon hybrid film in large-area flexible electronics.The excellent photoresponse of the graphene-carbon nanotube heterostrcture is closely related to the coupling interactions and charge transfer processes between the two materials.However,the fundamental charge transfer behaviors at the graphene-carbon nanotube interface remain less investigated due to the diverse electronic structures(chiralities)of single-walled carbon nanotubes(SWNTs).So we further address the limitation by complementing Raman spectroscopy with photocurrent probing,and unambiguously reveal the chirality-dependent characteristic in the graphene-SWNT heterostrctures.The graphene sheet was found to be p-type doping with(6,5)chirality-enriched semiconducting SWNTs(s-SWNTs)under dark conditions,and it will be possible to construct good interfacial potential barrier in a s-SWNT-graphene system,which is the major donation to the enhanced photoresponse of the all-cabon system.Different from this,when graphene is made to interface high purity metallic SWNTs(m-SWNTs),no bulit-in filed is present at the m-SWNTs-graphene system,and the weak photoresponse of the m-SWNTs-graphene hybrid device is governed by hot electrons originating in graphene.The results provide important design guidelines for controllable applications based on the all-carbon hybrid material.With the deep research of graphene,other Dirac semimetallic materials with similar electronic band structure(WTe2,Cd3As2,etc.)have aroused tremendous interest.In the particular case of WTe2,has been intensely investigated for condensed matter physicists due to its exceptional properties,such as the large non-saturating magnetoresistance,appearance of weak antilocalization and pressure induced superconductivity.However,it is a fundamental problem to develop various functional optoelectronic devices.Here,a heterostructure comprising of all semimetallic constituents,namely graphene and WTe2,is fabricated and the optoelectronic properties of the hybrid system is studied for the first time.The graphene-WTe2 phototransistor exhibits a pronounced photocurrent enhancement compared to the pure graphene device.Transport and photocurrent mapping measurements suggest that both photovoltaic and photothermoelectric effects contribute to the enhanced photoresponse of the all-semimetal hybrid system.The results help to enrich the understanding of physical mechanisms of novel hybrid interfaces and provide design guidelines for functional devices based on emerging 2D layered materials.In summary,optoelectronic properties and physical mechanisms of two novel important heterostructures based on low-dimensional materials,are systematically investigated.The results represent a significant reference towards facile fabrication of high-performance optoelectronic devices and will promote the development and optoelectronic applications of low-dimensional materials.