Investigation Of Transient Spectroscopy Of One And Two-dimensional Semiconductors

Since the birth of nanotechnology and nanomaterials,fruitful achievements and great progress have been made in this field.Various kinds of nanomaterials have appeared in the past few decades.As an important part of nanomaterials,nano semi-conductors have shown enormous potential in several fields of application,such as nano lasers,new type of solar cells,nano biosensor,new type of chemical catalyst and so on.On one hand,because its small size,nano semi-conductors are very suitable to be used to design and construct the materials/devices we needed as“building block”through the“bottom-up”pathway,which provides another way for us to achieve the functional devices in micro-nano scale.On the other hand,due to the quantum confinement and some other effects,nano semi-conductors possess many novel properties,such as special optical,electrical,thermal,magnetic and superconductive properties which are different from those of macro bulk materials.These properties provide not only the basic“building block”to design and manufacture many new functional devices,but also a platform to study the physics and mechanisms these properties.People began to use various methods and means to observe and study nano materials since the invention of scanning tunneling microscope in 1980s,in order to understand the whole picture of nano materials.Transient spectroscopy,or ultrafast spectroscopy,is an important means to study the behavior and dynamics of carriers in materials.Through the excitation of electrons in the material by ultrafast laser,and then observing the change of luminescence or absorption behavior and its dependence with time,we can understand the energy relaxation behavior and dynamics information of non-equilibrium carriers such as excited electrons in the material.These information can provide a solid theoretical basis for us to build appropriate functional devices by using the characteristics of materials.In this work,we committed to the application of transient spectroscopy into the study of nano semi-conductors.After process of semiconductor materials preparing into nano materials,many properties of the materials,such as band gap structure,carrier behavior,and carrier relaxation dynamics,will be different from those of bulk materials.Therefore,the observation and interpretation of the changes in properties will help us to understand and utilize these nano semi-conductors.In order to achieve this goal,we have designed and built two sets of transient spectroscopy measurement systems for nano semi-conductors,which can detect the transient fluorescence spectroscopy and transient absorption spectroscopy of nano semi-conductors,respectively.Then,we use these two devices to test and study the carrier behavior of one-dimensional gradient nanowire and two-dimensional transition metal dichalcogenide.In the study of one-dimensional gradient nanowire,we mainly focus on its specific asymmetric light propagation.The asymmetric light propagation behavior means the gradient nanowires can transmit optical signals in different directions according to different laws,which gives the gradient nanowire potential to manufacture logic devices.Through the measurement and analysis of the time-resolved fluorescence spectra of the propagated light signals in different directions and different distances,we have determined the differ ence between the time constants of the two kinds of propagated light signals and the optical signals at the excitation points,determined the sources of the different optical waveguide signals and analyzed the mechanism behind the asymmetric light propagat ion.In the study of two-dimensional transition metal disulfides,we focus on the change of the fluorescence behavior cause by the formation of trions.It is found that the Stokes shift of monolayer WS₂ is due to the radiative recombination of trions.However,on the glass substrate,the n-doping level of the substrate to the sample is higher,which is conducive to the formation of trions,hence the Stokes shift of monolayer WS₂ is larger.At the same time,we found that there are two different kinds of trions,one is singlet state and the other is triplet state.Through the above two studies,we observe and study the different properties of different nano semiconductor materials,and explain the mechanism behind their special properties,which provides theoretical guidance for us to utilize the special properties of this kind of nano semi-conductor.