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Computer Simulation On Glass Transition Of Nanomaterials, And Interface And Adsorption Properties Of Graphene

Posted on:2009-05-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z M AoFull Text:PDF
GTID:1101360272976449Subject:Materials science
Abstract/Summary:PDF Full Text Request
As a new reasearch field, nanotechnology, which bagan at the end of 1980s, now has significant influence on the fields of physics, chemistry, biology, material science, electronics and mechanics. On the other hand, non-crystal solid, usally also called glass, is one of the most intensive research subjects in the field of condensed matter. If the cooling process is quickly enough and the temperature is low enough, almost all of materials, including polymers, organic compounds and even metals, can transit into glasses. Recently, there are lots of attentions payed on glass transition of nanomatials, due to the size of the materials is located between bulk and cluster, and it is also on the upper limit of quantum mechanics calculation and lower limit of themodynamic analysis. Thus, these works are important for the applications of nanomaterials, especially for the phase transition theories of nanomaterials. Furthermore, these works are essential for the enhancement of understanding the relationship between the thermodyamic theories and quantum mechanics.Graphene, a single two-dimensional layer of graphite in a hexagonal structure, is the starting point for many nanographite devices and displays promising electronic properties. After the theoretical prediction of the peculiar electronic properties of graphene in 1947 by Wallace and the subsequent studies on its magnetic spectrum, it took half a century until the graphene could be experimentally fabricated and its anomalous quantum Hall effect was measured, which encourage numerous works on it now. Due to the instability of a freestanding graphene (it has an intrinsic three-dimensional structure or ripples), graphene used as devices is generally located on a substrate, such as on the common gate dielectricα-SiO2 substrate, which has significant effect on its electrical properties.On the other side, graphene related materials may be a solution for ultra-high sensitivity gas sensors. Similar to CNT, the working principle of graphene devices as gas sensors is based on the changes of their electrical conductivities induced by surface adsorbates, which act as either donors or acceptors associated with their chemical natures and preferential adsorption sites. Graphene is considered to be an excellent sensor material due to its following properties: (1) graphene is a single atomic layer of graphite with surface only, this can maximize the interaction between the surface dopants and adsorbates; (2) graphene has much smaller band gap energy, Eg, than CNT, hence, it has extremely low Johnson noise, therefore, a little change of carrier concentration can cause a notable variation of electrical conductivity; and (3) graphene has limit crystal defects, which ensures a low level of excess noise caused by their thermal switching.Now, computer simulation technology are being used widely in all kinds of fields, due to the following advantages: (1) the computer simulation is quick, so that the results can be gotten in short time; (2) it can do virtual experiments which can not be done actually; (3) it can provide many details which can not be obtained in actual experiments. Based on the above considerations, in this work, combing thermodynamic theoriesand classical molecular dynamics calculations, the thermodynamic and kinetic glass transition of nanomaterials were investigated. On the other hand, the first principle density functional theory method was used to calculate the graphene/SiO2 interface and the effect of electrical field on the atomic structure was studied. In addition, due to the advantages of graphene on gas sensor, the adsorption behavior of CO on intrinsic and Al doped graphene was calculated to prob the performance of the proposed sensor material. The detailed contents are listed as follows:1. The size dependent glass transition temperature Tg(w,D) of several polymer blend nano-films in miscible ranges are determined by computer simulation and the Fox equation where w is the weight fraction of the second component, D denotes the thickness of films. Tg(w,D) function of a thin film can decrease or increase as D decreases depending on their surface or interface states. The computer simulation results are consistent with available experimental results and theoretical results for polymer blend films of PPO/PS [poly (2,6-dimethyl- 1,4-phenylene oxide)/polystyrene] and stereoregular PMMA/PEO [poly (methyl methacrylate) /poly (ethylene oxide)].2. Based on Sutton-Chen many body potential function, several thermodynamic parameters of Ag are simulated by molecular dynamics. The parameters simulated are size dependences of Kauzmann temperature TK and melting temperature Tm, size and temperature dependences of melting enthalpy Hm and melting entropy Sm. The simulation results and results of thermodynamic theory models of TK and Tm shows a good agreement, which find that as the size of Ag particles decreases, TK and Tm functions drop. However, the ratio of TK and Tm of Ag nanoparticles is size independent.3. Segment dynamics of free-standing polystyrene (PS) films is determined by considering the temperature- and thickness-dependent number of styrene segments Nα(T,D) in the cooperative rearranging region (CRR). Under the help of Adam-Gibbs glass transition theory and molecular dynamics simulation, Nα(T,D) function is established and it decreases as D decreases or T increases. However, Nα[Tg(D),D] at the glass transition temperature Tg(D) is size-independent, which is consistent with the simulation results obtained by Donth′s method. Meanwhile, its relative temperature function Nα{[T-Tg(D)]/Tg(D)} is also size-independent. Therefore, Nα[Tg(D),D] function as a criterion for glass transition, which describes the physical nature of the glass transition, is similar to the vibrational amplitude in Lindemann′s melting criterion.4. The atomic structure of the graphene/α?SiO2(0001) interface was calculated using density functional theory. Simulation results indicated that atomic structure of the interface after relaxation was a function of the initial distance d0′between graphene and SiO2 surface. When d0′≥4.000 ?, obtained structures varied with d0′, and the interface interactions were weaker than that within graphite. While d0′< 4.000 ?, a minimum energy structure was obtained with the interface interaction stronger than that in graphite. Furthermore, the interface under electric field F with different intensities was also studied. Results indicate that the atomic structure of the graphene/α-SiO2(0001) interface has only a slight change under the condition of F≤0.02 au. As F reaches 0.03 au, the formation of C?O covalent bond on the interface is present, which would destroy the excellent electronic properties of graphene. Thus, there exists a maximum for F in application of the graphene.5. A principle of enhancement CO adsorption was developed theoretically by using density functional theory through doping Al into graphene. The results show that the Al doped graphene has strong chemisorption of CO molecule by forming Al?CO bond, where CO onto intrinsic graphene remains weak physisorption. Furthermore, the enhancement of CO sensitivity in the Al doped graphene is determined by a large electrical conductivity change after adsorption, where CO absorption leads to increase of electrical conductivity via introducing large amount of shallow acceptor states. Therefore, this newly developed Al doped graphene would be an excellent candidate for sensing CO gas. After that, the correlation of the applied electric field F and adsorption/desorption behaviors of CO molecule in the Al doped graphene was studied. The results indicate that the positive F reduces the adsorption energies Eads of the CO adsorbed onto the doped graphene, while Eads increases under the negative F. Furthermore, desorption commences when a large positive F (F≥0.03 au) is applied. Moreover, the best sensitivity of CO detection at F = 0.01 au is found. Finally, the thermal stability of interaction between the CO molecules and the Al doped graphene is studied with ab initio molecular dynamics calculation to reveal the adsorption/desorption behaviors of the system. Based on the results of the calculations, the adsorption?desorption phase diagram was established by the atomic thermodynamics and the temperature (T) dependent desorption timeτ(T) was determined with thermal desorption method. The results show that the optimal desorption temperature is 400 K. Meantime, the effect of T on atomic structure parameters and electrical properties were analyzed, and the results show that the greatest conductivity change before and after adsorption is at T = 400 K. Therefore, this sensor material has the best sensing performance with appropriateτand the biggest conductivity change at 400 K.
Keywords/Search Tags:glass transition, interface, adsorption, graphene, thermodynamics, computer simulation
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