Growth,Characterization And Electronic Devices Of Two-dimensional Organic Molecular Crystals

Since 2004,two dimensional atomic crystals represented by graphene have been extensively studied due to their unique physical properties and potential applications.However,two dimensional molecular crystals have attracted a few of attention.In recent years,significant efforts have been devoted to growth of organic semiconductor thin film,with high chemical purity,precisely controlled thickness,driven by the rapid growth of organic optoelectronics.But monolayer organic field effect transistor properties are limited in general,and show mobility below 0.1cm2V-1s-1.In this thesis,two dimensional organic molecular crystals can be grown on graphene and h-BN substrate via van der waals epitaxy method,making a great progress for device performance.The study was systematically carried out from epitaxial growth,characterization,device measurements of two dementional molecular crystals.The main works in this thesis are listed as follows:In chapter one,we give a brief introduction to packing mode,band structure and process method of organic semiconductor thin film,and introduce microscopic physical mechanism of carrier transport and influence factors affecting carrier mobility for organic field effect transistor.In chapter two,by using organic chemical vapor-phase deposition,we systematic researched the method of via van der waals epitaxy for C8-BTBT grown on graphene substrate.The growth process and crystal structure of two dimensional C8-BTBT molecular crystals were demonstrated by using AFM,HRAFM,HRSTM,Raman spectrometer,cross-polarized optical microscope and DFT calculation.The results show:(1)The crystals preferentially grew on graphene,in a layer-by-layer fashion with atomic smoothness.The thickness of the initial two layers(namely the interfacial layer,IL,and the first layer,1L)was?0.6 nm and?1.7 nm,respectively,the thickness of the subsequent layers(2L,3L,4L......)was?3 nm as bulk crystal.The nucleation sites of C8-BTBT molecule were formed easily at disorders and edges from the previous layers and from the substrate,likely due to their high surface energy.Then the growth initialed at certain nucleation sites and processed nearly isotropically as compact islands.In the growth process,the frequent interruption and ambient exposure of the sample did not significantly affect the growth,which suggested that the crystals were of pristine quality and stable against photooxidation in ambient.(2)We observed under HRSTM that the IL C8-BTBT molecules were packed in a rectangular lattice with d 1=2.52 nm and d2=0.66 nm in two orthogonal directions,consistent with the DFT result.The structure of the subsequent layers was studied by high-resolution AFM.We found that the crystal structures for both 1L and 2L were monolithic with herringbone-type packing as in bulk crystals.The lattice constants were a=6.24±0.25 A(6.69±0.13 A),b=8.31±0.05 A(7.89±0.25 A)for C8-BTBT crystals on graphene(BN).We did not observe any statistical difference of lattice constants between 1L and 2L.The C8-BTBT molecules in 1L seemed more inclined than bulk crystals due to the vdW forces decayed rapidly as r'6.(3)With careful control of the growth parameters,we repeatedly observed large-area uniform 1L C8-BTBT single crystals up to?80 ?m in size on defect-free graphene.In addition,we successfully achieved the growth of 2D molecular crystals on CVD graphene substrate with centimeter scale as well as patterning capability.In chapter three,we demonstrate two-dimensional quasi-freestanding Cg-BTBT molecular crystals for high-performance organic field-effect transistors electrical properties,including two kinds of structure,vertical OFET(graphene electrode-C8-BTBT molecular crystal-Au electrode)and planner OFET(Au electrode-C8-BTBT molecular crystal-Au electrode).Vertical OFET exhibited transistor properties and diode-like rectifying behavior.And we implemented logic gates by intergrating two vertical OFET.Under forward bias,holes were thermally activated over a Schottky barrier ?SB at the metal-semiconductor interface in the high temperature regime,then accessing the organic semiconductor layers and collected by source electrode.Under reverse bias,the thermionic emission current became exponentially small,and the charge transport was dominated by tunnelling through the organic semiconductor layers.The device showed on/off ratio?106 at room temperature.At last,we fabricated planar OFETs with 1L C8-BTBT crystals grown on insulating BN,with the saturation voltage down to?1V.And the devices show carrier mobility up to 10 cm2/V-1 s-1,much higher than previously reported values for monolayer OFETs.The devices exhibited some features of ideal OFETs.In chapter four,we study the structural origin of the high carrier mobility of a molecular monolayer on boron nitride.Firstly,we simulate the growth mechanism of C8-BTBT molecular crystal from theory.Secondly,we demonstrate the height of the out-of-plane roughness or step on substrate is a critical parameter that determines the growth of the IL C8-BTBT molecular crystal.The steps with height larger than 0.6 nm could act as the C8-BTBT nucleation center.Of course,high-density of such roughness could induce disturbance on the growth of C8-BTBT IL and is harmful to grow large-area single-crystal C8-BTBT IL.In chapter five,we make a summary about this thesis and outlook for the next research.