Local phenomena at grain boundaries:An alternative approach to grasp the role of oxygen vacancies in metallization of VO_(2)

Vanadium dioxide(VO_(2))undergoes an insulator to metal transition(MIT)and an accompanied phase transition from a monoclinic(M)structure to rutile(R)structure near room temperature,forming the basis for many VO_(2)-based functional devices.The MIT transition of VO_(2)and the functionality of VO_(2)-based devices can be controlled by a variety of chemical and physical stimuli.With these external stimuli,defects,such as oxygen vacancies,are often inevitably introduced.However,due to the VeO systeminduced challenge to synthesize stable VO_(2)with different oxygen vacancy concentrations,the impact of oxygen vacancies on the resistance and transition of the VO_(2)is not fully understood.Oxygen vacancy,as one of the typical defects in VO_(2),is expected to concentrate at grain boundaries,and hence a concentration gradient of oxygen vacancies may exist between the grains interior and the boundaries,and this suggests a possibility to study the effects of oxygen vacancies on the transition of VO_(2)by probing local phenomena at the grain boundaries.For investigating local phenomena at the grain boundaries,Scanning Probe Microscopy(SPM)techniques are effective,which allows probing the structure and various properties at the nanoscale.In this work,a series of SPM techniques,including Atomic Force Microscopy(AFM),conductive-AFM(c-AFM),Electrochemical Strain Microscopy(ESM),and Kelvin Probe Force Microscopy(KPFM),are employed to measure variations of the surface structure,the resistance,the oxygen vacancy concentration,and the work function between the grain interior and the grain boundary.It has been demonstrated that,for most cases,both the resistance and the work function are lower at the grain boundaries as a result of the accumulation of oxygen vacancies at those positions.In addition,the resistance change induced by the electric field has been observed in the deposited VO_(2)thin films,which may be associated with the generation/annihilation of the oxygen vacancies,rather than charge injection.This work has demonstrated the effects of oxygen vacancies in the transition of VO_(2)by probing the local phenomena at grain boundaries,also provided a new insight into the resistance change of VO_(2)under an electric field.

Effects of oxygen and moisture on the I-V characteristics of TiO_(2) thin films

Current-voltage(I-V)characteristics well reveal the resistive switching performance of materials promising for the next-generation memory-resistance random access memory(ReRAM).It has been observed that the atmospheric environment can affect the resistive switching performance,but the origin of this effect is still under debate.Conductive Atomic Force Microscopy(c-AFM)is widely used to study the resistive switching performance because of its capability to realize the resistive switching at the nanoscale that is becoming attractive as the miniaturization of memory devices.This study therefore aims to understand the effects of oxygen and moisture on the I-V characteristics of the TiO_(2)thin film by performing c-AFM measurements in ambient air,synthetic air,and argon gas.It is found that the oxygen in the environment can reduce the set and the reset voltages for the resistive switching,and it can also reduce the resistance at the low resistance state(LRS).Where the moisture in the environment can increase the set and reset voltages,and increase the resistance at LRS.These effects of oxygen and moisture in the environment can be attributed to the modification of the effective electric field during the resistive switching processes,which have been further confirmed by Kelvin Probe Force Microscopy(KPFM)measurements.In addition,it is found that the local ionic dynamics of TiO_(2)during the resistive switching are strongly dependent of the environments by performing the FORC-IV(First Order Reversal Curve-Current-Voltage)measurements in the three gas environments.Results in this work can provide a new perspective on the effect of environments on the resistive switching of materials,that is,the modulation of the effective electric field due to the adsorption of oxygen and moisture under the c-AFM tip.

Variation of contact resonance frequency during domain switching in PFM measurements for ferroelectric materials

Piezoresponse Force Spectroscopy(PFS)is a powerful technique widely used for measuring the nanoscale electromechanical coupling of the ferro-/piezo-electric materials.However,it is found that certain nonferroelectric materials can also generate the“hysteresis-loop-like”responses from the PFS measurements due to many other factors such as electrostatic effects.This work therefore studies the signal of the contact resonance frequency during the PFS measurements.By comparing the results from ferroelectric and non-ferroelectric materials,it is found there are distinct differences between these two types of materials in the variation of the contact resonance frequency during the PFS measurements.A momentary and sharp increase of the contact resonance frequency occurs when the domain is switched by applying the DC bias,which can be regarded as a unique characteristic for the ferroelectric materials.After analyzing the reliability and mechanism of this method,it is proposed that the contact resonance frequency variation at the coercive bias is capable to differentiate the electromechanical responses of the ferroelectric and non-ferroelectric materials during the PFS measurements.