Tuning The Charge Transport In Metal-Organic Frameworks

Metal-organic frameworks(MOFs),which are formed by association of metal cations or clusters of cations(“nodes”)with multitopic organic bridging ligands(“linkers”),are a fascinating class of crystalline hybrid materials offering unique chemical versatility and extraordinary degree of variability in their functionalities.For instance,the inherently ultrahigh porosity of up to 90% free volume and the controlled-access to and from the enormous internal space of the porous frameworks render MOFs various applications in gas storage,chemical sensing,catalysis,drug delivery,ferromagnetic material and etc.MOFs also can be taken as the highly ordered inorganic-organic hybrid system.In principle,the carrier densities and mobility of MOFs may better than the disordered polymer.Recent years,deliberate tuning of the electronic properties of MOFs to design the stable electronic devices is beginning to attract the attention from material scientists.More importantly,MOFs possess the features of both inorganic and organic material,which may provide alternative strategy for the construction of flexible electronics.Flexible electronics is a newly-developing electronic technique,which has shown great potential applications in the field of information,energy and health care etc.The most fundamental features of them is the functional electronic devicesare fabricated on the flexible substrates for the bending,stretching and twisting applications such as wearable electronics,e-skin and so on.However,inorganic materials are notorious for their brittleness.They tend to fracture under a tensile strain of about 1%,so it is impractical to incorporate them directly into flexible electronic devices.Fortunately,the organic materials may break the bottleneck due to the flexible chain structure.While organic materials show poor performance and thermal stability.Maybe,the MOFs-based flexible electronics can work miraclesIn this thesis,we focus on the resistive switching effect and piezoresistive effect of metal-organic framewoks and successfully modulate the resistance of MOFs using electrical field and strain.Also,we discuss the charge transfer process in MOFs.The works we performed as follows:1.We have fabaricated a new MOF,denoted as RSMOF-1,which exhibited stable,repeatable and uniform resistive swithching effect.The voltage of Reset and Set ranged in 1~2V/-1~-1.5V and 7.5~8.5V/-7.5~-8V,respectively.And the On/Off ratio is about 30,the power consumption is ~0.4 nW.Moreover,we have observed a stable ferroelectricity in RSMOF-1,which originated from the “ferroelectric ice” structure between the guest water and the frameworks.The swing and filp of the N···H-O···H-N induced by the electrical field is also the reason of resistive switching effect in RSMOF-1.At last,we have shown the dynamic interaction between the guest molecular-water and the pendant group –NH2 of the frameworks based on the Ab Initio Molecular Dynamics Simulation.The discovery of the resistive switching and ferroelectric behaviors in the MOF material as well as the improvements in our understanding and control of how MOFs transport charge and interact with the macroscopic world are bridging the gap between these fascinating materials and new devices and applications.2.A high-quality HKUST-1 nanofilms have been fabricated directly on flexible substrates using a modified LPE approach,which exhibit reproducible resistive switching effect.The memory switching parameters of HKUST-1 are highly uniform,with the narrow distribution of RHRS(HRS resistance,400 ± 15.1 ?),RLRS(LRS resistance,31.9 ± 0.51 ?),Vset(set voltage,0.655 ± 0.013 V)and Vreset(reset voltage,-0.51 ± 0.011V).These parameters is better than the present materials based on the evaluation of Weibull function.Also,the resistance switching between the ON and OFF states is highly stable in the working temperature range of 10 K to 400 K,which is rarely observed in organic based materials.Moreover,Au/HKUST-1/Au/PET also shown the uniform and stable resistive switching effect,which can be sustained under the strain of as high as 2.8%,and over the wide temperature range of-70 °C to +70 °C.The mechanical flexibility and environmental stability well meet the requirements for wearable application under earth surface conditions,thus providing promising material candidates for the development of flexible or even wearable information storage teqechniques.At last,we have discussed the mechanism of the resistive switching effect in HKUST-1 using the C-AFM and XPS.With the high electric field applied onto the Au/HKUST-1/Au sandwiched structure,Cu2+ ions localized at the crystalline defects can be dislocated from the BTC linkers,driven into the top Au layer and reduced to Cu atoms.The negatively charged vacancies in the HKUST-1 nanofilms are relatively less stable,and the carboxylic groups can be removed from the aromatic linkers upon Joule heating and emitted as carbon dioxide through the top electrodes.The pyrolysis of the trimesate linkers then may result in the coupling of the neighboring benzene rings and the subsequent formation of sp~2-hybridized carbon-rich channels.3.Using the AFM and SEM,we have confirmed the growing process of MOF CuTCA nanofilm: nucleation,expansion and growth,nanofilm formation,which is the guidance for the fabrication of other MOF nanoflims.Then,we have studied the intrinsic piezoresistive effect of I2@CuTCA,which could endure a strain of about 4.3% and exhibit a large On/Off ratio about ~80.The possible mechanism of piezoreisitive effect in I2@CuTCA is the deformation of the frameworks.And,the applied strain changes the activation energy of the hopping charge which also contributed to the change of the conductance.The piezoresistive I2@CuTCA can be used as the switch element and strain-gated transistor which exploits the applications of MOFs toward the electronics industry such as wearable electronics,sensor,logical circuit and MEMS/NEMS.