Nanoscale Structures And Physical Properties Of 2D SnSe Materials

The advent of graphene-based two dimensional materials（2D）have revolutionized the present material research fields.More and more new 2D materials have been paid great attentions due to their unique chemical and physical properties and promising applications in the future.2D SnSe material,as one of 2D chalcogenide and transition metal oxides,is an important p-type semiconductor with a black-phosphorus-like crystal structure,and exhibits excellent optoelectronic,thermoelectric,spin transport,multiferroic,and piezoelectric properties in applications to photodetectors,storage switching devices.Up to now,few work has been involved with their nanoscale physical properties（local electrical,thermal,thermoelectric,etc）in response to the external fields（stress,temperature,optical etc）,which is very crucial for its next research and development.Atomic force microscope（AFM）-based scanning probe microscopies have now become powerful tools for imaging nanostructures and characterizing properties.In the present thesis,AFM-based nanoscale electrical,thermal,elastic,thermoelectric mode nanotechnology were employed to perform studies of nanoscale electrical,thermal,thermoelectric,elastic properties of 2D SnSe material under the different external fields.Some important conclusions were obtained as follows:（1）A in-situ nanoscale heating technology with the AFM conductive tips weredeveloped and used to perform thinning thickness of 2D SnSe nanosheets.Nanoscale thermal and thermal stresses were firstly found to induce in-situ local phase transition of SnSe into SnSe₂.（2）Nanoscale thermal conductivity and Seebeck coefficient of 2D SnSe materials werein-situ studied by scanning thermoelectric microscopy.Local thermal propertieswere found to be thickness dependent,2D SnSe nanosheets demonstrate adecreasing thermal behavior with the reduction of thickness in the range of 6 nm to40 nm.The unique thermal behavior is ascribed to the non-harmonic interlayercoupling along the armchair direction of SnSe crystal structure.It was found out that the Seebeck coefficient of 2D SnSe is anisotropic.（3）Nanoscale electric behaviors of 2D SnSe materials were investigated by conductiveatomic force microscope combined with the local electric fields,stresses,temperature and light fields.Nanoscale I-V characteristics showed complexthickness effects,and both local stress and heating temperature via the AFM tip cansignificantly change the Schottky barrier distributions,leading to the change of theoriginal semiconduct state into the metal state.The red light demonstrated littlechange of local I-V curve,while the purple light was found to play an important role of modulating carrier transport distribution.（4）Nanoscale elastic properties of 2D SnSe was in-situ characterized by nano-elasticmicroscopy.Based on the AFM force curve spectroscopies and the classical Hertzcontact model,High throughput nanoscale elastic imaging and quantitativecharacterizing of 2D SnSe nanosheets were successfully achieved simultaneously.Nanoscale Young’s modulus are averaged to be in the range of 45～55 GPaproviding a strong foundation for next similar work about quantitative elastic characterization of 2D materials.In summary,nanoscale electrical,thermal,thermoelectric and elastic properties were characterized by advanced scanning probe microscopy（conductive mode,thermal mode,elastic mode,thermoelectric mode,etc.）.The present results in this work provide new insights into the nanoscale physics of two dimensional SnSe materials,and also extend new application of advanced scanning probe microscopy.