Preparation And Optoelectronic Properties Of Copper Hexadecafluorophthalocyanine Nanoflake And Its Heterostructures

Low grain boundary density and long-range ordering within two-dimentional(2D)organic crystals lead to the efficient charge dissociation,injection and transport,as well as rapid response to stimuli,ensuring their applications in fields of high-performance electronics and logic circuits.Air-stable,photosensitive copper hexadecafluorophthalocyanine(F₁₆CuPc)is a promising n-type semiconductor for organic electronics and optoelectronics.However,there are several factors that restrict the performance of F₁₆CuPc.The poor crystallinity and bulk nature of F₁₆CuPc thin films,nanowires and micro-/nano-ribbons hinder the efficient charge transport,and the strong Fermi level pinning(FLP)effect and large Schottky barrier between gold electrodes and F₁₆CuPc largely limit efficient charge injection and the resultant device performance.Based on the analysis above,high quality 2D F₁₆CuPc nanoflakes are obtained by facile mechanical exfoliation from bulk crystals prepared by physical vapor transport(PVT),and more efficient charge transport and excellent UV-NIR photoresponse are found for 2D F₁₆CuPc nanoflakes,making them promising candidates for high performance nanoelectronics.In addition,by introducing van der Waals(vd W)contact,FLP effect is strongly surpressed and the Schottky barriers become tunable,thus the electrical and electronic performance of2D F₁₆CuPc nanoflake are further improved.Furthermore,by fabricating Ge As thin film with narrow bandgap and 2D F₁₆CuPc nanoflake based vd W tunneling heterostructure,the sensitive spectral range of F₁₆CuPc has been widen to long-wavelength infrared(LWIR),and high performance visible and LWIR phototransistor with low dark current,and the charge transfer mechanisms within the heterostructure have been revealed by electrical and KPFM characterizations.In this work,the charge transport and electrical contact behavior of F ₁₆CuPc nanoflakes,mechanically exfoliated from needle-like single crystals,are probed by analyzing the temperature-dependent carrier mobility and conductance,where the multi-trap/release and band-like transport mechanism govern the charge transport at different temperature ranges and carrier densities.F₁₆CuPc nanoflakes based field-effect transistors(FETs)exhibit high on-state current and ON/OFF ratio,one-order magnitude higher than that of reported F₁₆CuPc nanowires,thin films and nanoribbons.Besides,F₁₆CuPc nanoflake-based phototransistors exhibit attractive photoresponse performance in the spectral range of 300?750 nm even at quite low operating source-drain voltage(?1 V),with maximum photoresponsivity of 19 A/W,detectivity of 8×10¹² Jones,and fast response speed of 36 ms.Such excellent electrical and electronic performance is attributed to the single-crystalline characteristic of nanoflakes,and the resultant efficiently exciton diffusion and charge transport.By carrying out temperature-dependent transport and Kelvin probe force microscopy measurements,we demonstrate that the introducing of 2D metallic1T-Ta Se₂ and multilayer(ML)graphite with matched band-alignment as electrodes for F₁₆CuPc nanoflake FET leads to enhanced field-effect characteristics,which are attributed to the significantly suppressed FLP effect and appropriate band alignment at the nonbonding vd W hetero-interface.Compared with evaporated Au contact,the current density of the device with 1T-Ta Se₂(ML graphite)contact has been improved by a factor of 12(46),the Schottky barrier height is lowered to 110(140)me V,and the contact resistance has reduced by a factor of 15(3).Moreover,by taking advantage of the improved contact behavior with 1T-Ta Se₂ and ML graphite contacts,the optoelectronic performance of F₁₆CuPc nanoflake-based phototransistors has been drastically improved,with a maximum photoresponsivit y of 387 A/W and detectivity of 3.7×10¹⁴ Jones for 1T-Ta Se₂ contact and 588 A/W and detectivity of 1.2×10¹³ Jones for ML graphite contact at quite a low source-drain voltage of 1 V,which is more competitive than those of the reported organic photodetectors and phototransistors.By employing GeAs thin film with narrow bandgap and 2D F₁₆CuPc nanoflake based organic/inorganic heterostructure,the sensitive spectral range of F₁₆CuPc has been widen from UV-vis to LWIR,and high performance visible and LWIR phototransistor with low dark current,high detectivity and strong gate-dependence has been achieved.Phototransistor based on F₁₆CuPc/Ge As heterostructure exhibits outstanding performance with maxium photoresponsivity of 110 A/W and specific detectivity of 9.2×10¹² Jones under 532 nm illumination.Ultra-low dark current and gate-tunability of photoresponsivity and detectivity of the heterostructure have been achieved under 1550 nm illumination.Under forward bias,photoresponsivities and detectivities have been continuously tunable from-1.6 to 3.1 A/W and-1.1×10¹⁰ to1.5×10¹⁰ Jones,respectively.Under reverse bias,maxium photoresponsivity of 19.1A/W and specific detectivity of 1.3×10¹³ Jones have been obtained.Furthermore,by analizing surface distributions of the heterostructure at varying source-drain bias and gate bias before and under illumination,the band alighments of the heterostructure have been schemed,and the charge transfer mechanisms within the heterostructure have been revealed.Tunneling facilitates efficient charge transfer at the heterostructure interface,which results in the high performance of visible and LWIR dectection.