| Two-dimensional（2D）materials are a new material catalogue having atomic thickness in 2D direction.They exhibit many unique optical,electrical and mechanical properties due to the limitation of dimensions.After graphene,transition metal chalcogenides（TMDs）with similar structures have attracted wide attention.TMDs materials possess good flexibility and various conductive properties,such as metal,semiconductor,insulator and so on.So they have a broad applied prospect in devices of electronics,optoelectronic,sensor and other fields.Molybdenum disulfide（Mo S2）,as an important member of the TMDs family,with monolayer band gap of about 1.8 e V,is a type of ideal semiconductor.Therefore,the application of Mo S2 will have promised potential in thin film transistor（TFT）and has become a hot topic in the community of research field.At present,how to prepare high-quality Mo S2 flakes and to clarify the characteristics of semiconductor and interface in Mo S2-TFT are the urgent issues in this field.This dissertation focuses on these issues and conducts studies as following:1.The preparation of high-quality Mo S2 flakesDue to the small-area and low-yield of Mo S2 flakes by classical mechanical exfoliation,it is a main challenge for the process of TFT devices.Two kinds of preparing processes for high-quality Mo S2 flakes are adopted in this study based on improved mechanical exfoliation as follows:（1）The preparation of Mo S2 flakes based on solvent-processWe proposed a new dissociation process by introducing solvent-process between Mo S2 bulk and Si O2 substrate.In this process,four Mo S2 samples are prepared on Si O2 substrates,and the acetone,isopropanol,ethanol and deionized water is introduced to form solvent-layer before exfoliation,respectively.The spectral properties are recorded in according with that of Mo S2 flakes with few layers by traditional exfoliation.More importantly,this improved process brings large-area and high-yield.Two probable exfoliation-mechanisms are assumed to responsible for the high-quality Mo S2 flakes:First,the outward capillary force comes from solvent concave,and the adhesion force is generated from the solvent concave between two materials.These two forces effectively increase the contact area of Mo S2 and Si O2 so that the van der Waals force in the interface is enlarged.Second,the work of adhesion increases obviously at Mo S2interface which is processed by solvent.It actually promotes the adhesive ability between Mo S2 and Si O2.（2）The preparation of Mo S2 flakes based on thermal treatmentWe explore the ideal operated process in order to clarify the influence of thermal treatment on the exfoliation of Mo S2 bulk.The process confirmed through comparative experiments is as below:heating the samples at 110℃for 120 minutes,and then getting the Mo S2flakes from bulk after annealing for 10 minutes.We found that the area and yield of flakes are better in this way according to the statistics.Meanwhile,a monolayer flake about 200μm×100μm was found by AFM.The mechanism of thermal treatment may lie in following two aspects:First,this process can reduce the water molecular in the interface.Thus,the increased contact area between Mo S2 and Si O2 results in the enhancement of the Van Der Waals force.Second,the annealing for a short time after thermal treatment can lower the temperature so as to restrain the molecular movement.This mechanism may strengthen the Van Der Waals force at the interface and make the internal force relatively small before the temperature falls.Therefore,the resultant force that occurs at the boundary of Mo S2 facilitates the dissociation of bulk,and is benefit to achieve monolayer Mo S2.2.The preparation and semiconducting properties of Mo S2-TFTWe prepared Mo S2-TFTs based on the drain-source electrodes of Au,Ag,Al,Cu,and carried out the study on semiconducting properties of devices as following:（1）The properties of metal-semiconductor contact in Mo S2-TFTs with different metallic drain-source electrodes.First,we characterized the four Mo S2-TFTs and obtained the field-effect mobility,on/off current ratio,threshold voltage,maximum saturated current and other important parameters from output,transfer curves of the devices.From the data above,the calculated effective barrier heights of the metal-semiconductor contact in TFTs based on electrodes of Au,Ag,Al,Cu,are 10.487 e V,10.501 e V,10.606 e V and10.448 e V respectively according to the theory of thermionic emission.We can come to a conclusion that there are mismatches between the barrier heights and the metallic work functions in these devices.We confirm that Fermi level pinning（FLP）effect exists in the metal-semiconductor contact following the Schottky-Mott rule.FLP is mainly caused by the surface states of semiconductor layer in the devices.As a result,the on-state current of devices is kept at the same order of magnitude,and is less relevant to metallic work function of drain-source electrodes.（2）The stability of Mo S2-TFT based on the drain-source electrodes of AgThe stability of Mo S2-TFT based on the drain-source electrodes of Ag is studied.The devices were placed in air for 0,48,120,and 216 hours respectively.We found that the on/off-state current,field-effect mobility of the devices raised obviously.These phenomena may be interpreted as following:Sulfur vacancies in Mo S2 are ease to absorb the oxygen molecules in ambient and turn into peroxide radical.Herein,the negative charges accumulate and form the extra channel current in electric field.The operated mechanism leads to the decline of on/off-state current and the rise of threshold voltage.3.The influence of interfacial modification on the performance of TFTTwo kinds of organic materials,p-6p and OTS,are introduced for interfacial modification in TFT to enhance the performance of devices.The main works are as below:（1）Ambipolar transport behavior and stability of Ti OPc-TFT based on the interfacial modification of p-6P and OTSp-Sexiphenyl（p-6P）and Octadecyl trichlorosilane（OTS）are used as insulator-modification materials of Ti OPc（p-type）TFTs for the purpose of exploring the ambipolar transport in devices.We found that the transfer efficiency of electrons was prominently promoted.Thus,we observed ambipolar transport behavior from our devices.The phenomena may be attributed to the generation of highly-ordered thin films and the inhibition of OTS on hydroxyl that are facilitated by p-6P.Moreover,the ambipolar transport behavior had retained in air for seven days,and the on/off ratio of holes were even better than before.So we confirm that the modification may improve the stability of devices in ambient as well.（2）Performance improvement of Mo S2-TFT based on interfacial modification of OTSWe adapted OTS as interfacial-modification material of insulator layer to estimate its effect on the performance of Mo S2-TFT in this study.The characterization of device modified by OTS are showed below:The field-effect mobility,threshold voltage,on/off current ratio,off-state current,on-state current at VDS=10 V were48.52 cm2/Vs,-0.91 V,2.9×104,8.05×10-3μA,233.19μA.The maximum saturated current at VG=30V was 463.49μA.The off-state current and threshold voltage are reduced by three and two orders of magnitude respectively comparing to the device without modification.The performances are evidently better than the unmodified device in general.We consider that the improvement may lie in the reduction of peroxide radical formed from oxygen molecules absorbed by sulfur vacancies and inhibition of the accumulation of negative charges in channel.And then the off-state current and threshold voltage predictably decline based on the above mechanism.In addition,the modification effect mentioned above still existed 9 days after the device was prepared.In summary,we focus on Mo S2 and its TFTs to carry out the studies on the preparation of flakes and devices,the semiconducting properties and interfacial modification in TFTs.In the dissertation,we explorer the dissociated process for Mo S2bulks and obtain large-area,high-yield flakes successfully.Besides,the screening effect of FLP caused by surface states in Mo S2 is clarified and the influence of sulfur vacancies on the electrical performance stability of Mo S2-TFT is proposed.In addition,we achieve ambipolar transport and improvement of electrical performance in TFTs by interfacial modification.The studies may further improve people’s cognition of Mo S2 flakes and TFTs.All the works above will show good practical significance and positive promoting action around the application of 2D materials in high-performance electronic devices. |
