Growth Of Ultrathin Functional Crystals By Micro-spacing Sublimation

With the rose of flexible electronics and people’s desire for electronic devices with higher integration levels that can continue Moore’s Law,the low-dimensional semiconductor crystal have become one of the hotspots of scientists from all over the world.As the functional layers of electronic devices,these low-dimensional crystals are generally composed of single or several atomic/molecular layers,the force between the layers is van der Waals force,and the thickness is in the nanoscale range.So,the new electronic devices have more possibilities in device structure,preparation method and application field.Moreover,compared with bulk crystals,electrons in low-dimensional crystals are bound in one-dimensional or twodimensional space,forming many properties that are completely different from bulk crystals,and have very novel application prospects in many fields.These unique properties of lowdimensional crystals are closely related to their thickness.But the controllable growth of lowdimensional crystals is still a difficult problem,and the research on the relationship between crystal thickness and material properties and device performance is also in the exploratory stage.This dissertation focuses on novel controllable growth methods for organic and inorganic lowdimensional crystals.Two-dimensional crystals of organic semiconductors generally belong to molecular crystals,in which structure is the organic molecules as the basic unit arranged in a long-range order in the two-dimensional plane,and the interaction between organic molecules is weak force such as hydrogen bond,π-π interaction and van der Waals force.Most organic crystals are difficult to grow high quality crystals with a thickness of only a few molecular layers due to the poor crystallizability.In particular,some high-performance conjugated small-molecule organic semiconductor materials are insoluble in any organic solvent,and are difficult to grow by solution methods.Moreover,the crystals growth by the solution method can’t reflect the organic semiconductor’s intrinsic properties due to the residual solvent in the crystal lattice.Therefore,the one of the difficulties in the research of ultrathin organic functional crystal is how to grow ultrathin organic functional crystals by vapor method to maximize device performance and meet the requirements of high-performance device applications.By comparison,two-dimensional inorganic crystals are generally composed of atoms or ions as basic units,which is different from organic crystals.Two-dimensional inorganic crystals have close relationship between structure and properties,and different thickness maybe have different or diametrically opposite characteristics,such as CrI3 crystals with single-layer thickness is ferromagnetic,while the thickness with double layers is antiferromagnetic.Therefore,the preparation of inorganic two-dimensional crystals with controllable thickness is of great research significance.In summary,the aims of this thesis are to grow ultrathin functional crystals.Firstly,we are starting from the growth of ultrathin organic conjugated small molecule crystals,combined with the micro-spacing sublimation method previously invented by our research group.By adjusting the distribution in the bottom substrate of the materials and growth parameters,we have regulated the growth of organic semiconductor crystals with different thicknesses,and deeply explore the physical issues behind the effect of thickness reduction on device performance.Moreover,we use the experience of growing ultrathin organic single crystals to explore the growth of ultrathin lead halide crystals and other inorganic crystals,and studied the mechanism of the perovskite process and the thickness control of inorganic crystals.The main research contents are as follows:1.Through the micro-spacing sublimation crystal growth method invented by our research group,we studied the relationship between the mean free path of sublimation molecules and the transmission distance,and found the dependence between the morphology of the source materials and that of the grown crystals.By controlling the distribution of source materials on the bottom substrate to control the transport of vapor molecules in the plane and vertical directions,the spreading method of source materials was improved through multiple comparisons,and finally the ultrasonic spray method was determined.Thus,the DNTT and pentacene ultrathin organic semiconductor crystals with a thickness of only a few of molecular layer were obtained.Based on these organic ultrathin crystals,field effect transistor devices were fabricated.The average carrier mobilities of the DNTT and pentacene devices were 6.08 cm2 V-1 s-1 and 2.39 cm2 V-1 s-1,respectively.And,the devices also exhibited high reliability factor(68%～92%),low threshold voltage(～83%of the devices within 5 V),and small subthreshold swing(≥60 mV/decade).The reasons for the excellent performance of the device are attributed to the reduction in bulk resistance due to the reduction of the crystal thickness,and a high-quality contact interface between the dielectric and semiconductor layers.2.The micro-spacing sublimation method was extend from organic crystals to inorganic crystals.We researched the thickness regulation of inorganic lead halide crystals,and the effect of crystal thickness on the process and morphology of halide vapor conversion to perovskite.we have successfully grown ultrathin lead iodide crystals with thicknesses ranging from a few nanometers to several hundreds of nanometers.The lead iodide crystals have good orientation and high crystallinity certified by the characterization of crystal structure and quality.Then,we observed the conversion process of these individual lead iodide single crystals with different thicknesses reaction with methyl iodide(MAI)vapor to form lead methyl iodide(MAPbI3)by a combination inspection of optical microscope,multichannel confocal fluorescence microscope and atomic force microscope.The effects of the conversion temperature and duration,and specially the crystal thickness on the transformation dynamics and morphology of the formed MAPbI3 are studied.Finally,we found that the transformation process was a single-crystal-to-polycrystal process,following a deconstruction-reconstruction mechanism.This work clarifies the formation mechanism of the conversion process.3.Two-dimensional inorganic crystals of chromium trichloride(CrCl3)have been successfully growth by micro-spacing sublimation method.Two-dimensional CrCl3 crystals exhibit layer-dependent ferromagnetic and antiferromagnetic properties,and their few-layer crystals can generally only be prepared by artificial exfoliation.Aiming at the problem that such materials are unstable in the air containing oxygen,we have improved the high-temperature micro-spacing sublimation equipment in a nitrogen atmosphere.The CrCl3 crystals with only two or three layers of thickness were successfully grown by controlling the temperature gradient and the surface properties to regulating the number of nucleation and the growth rate of the top substrate.Applying this strategy to other inorganic two-dimensional materials,ultrathin Bi2Se3 crystals which is topological insulator and ultrathin Bi2O2Se crystals which is infrared Semiconductor were successfully grown.This work provides a new and simple method for in-situ growth of inorganic 2D crystals with controllable layer number.The research results of this paper were selected by the American Chemical Society ACS Editors’ Choice,and ranked first in the 2020 crystal growth paper reading and download rankings.Chemical&Engineering News,a world-renowned news magazine in the field of chemistry and chemical engineering,September 2020 Reported with the title of "Sublimely simple method makes ultrathin organic crystals",interviewed many internationally renowned experts in the field,and believed that this new method "can make the preparation process simpler and lower cost".