【摘要】：材料與人們的生活息息相關(guān),并推動著人類社會的發(fā)展。歷史上每一次重大的經(jīng)濟和社會變革幾乎都能在背后找到材料的影子。例如,半導(dǎo)體材料的發(fā)現(xiàn)和大規(guī)模應(yīng)用,導(dǎo)致了人類社會步入微電子時代,至今使人受益。然而自然界存在的材料有限,已無法滿足人們?nèi)找嬖鲩L的物質(zhì)需求。這就需要人們基于現(xiàn)有的材料,利用各種物理或化學(xué)手段設(shè)計出具有特定功能的新型材料。在實驗學(xué)家看來,這是一個不斷嘗試不斷修正的艱巨任務(wù),在這一過程中,需要投入大量的時間和精力,而且不可避免的會導(dǎo)致實驗資源的浪費。 計算量子化學(xué)的發(fā)展為這一困境帶來了福音。利用第一性原理計算對材料的性質(zhì)進行預(yù)測,根據(jù)預(yù)測的性質(zhì)對材料進行初步篩選,然后再進行實驗驗證,將極大地提高實驗學(xué)家工作的效率,縮短材料設(shè)計的周期。本論文的目的在于介紹我們基于第一性原理計算,對自旋電子學(xué)材料和光解水催化材料進行功能導(dǎo)向設(shè)計的工作。這兩個領(lǐng)域看似毫無關(guān)聯(lián),實際都是日益嚴重的能源和環(huán)境危機促使的。前者的目的在于以更低的能量成本獲得更高的速度,后者則是尋找潔凈的可持續(xù)能源,即太陽能制取氫能,替代現(xiàn)有的化石能源。 自旋電子學(xué)基于電子自旋進行信息的傳遞、處理與存儲,具有目前傳統(tǒng)半導(dǎo)體電子器件無法比擬的優(yōu)勢,比如說運行速度更快,集成度更高,耗能更低,因而成為近年來人們研究的熱點。然而,自旋電子學(xué)面臨著三大挑戰(zhàn)：自旋的產(chǎn)生和注入,自旋的長程輸運,以及自旋的調(diào)控和探測。這些問題的解決將主要依賴于尋找具有特定性質(zhì)的自旋電子學(xué)材料,例如磁性半導(dǎo)體材料,半金屬材料等。盡管已經(jīng)有不少自旋電子學(xué)材料被相繼提出,但是他們距離實際應(yīng)用還存在較大的距離。其中原因包括自旋熱翻轉(zhuǎn)導(dǎo)致半金屬性被破壞,磁有序溫度低于室溫,合成困難或不易控制等。我們著眼于通過第一性原理計算設(shè)計具有特殊功能的新型自旋電子學(xué)材料,以及尋找在室溫環(huán)境下可用的自旋電子學(xué)材料,為自旋電子學(xué)器件的合成和應(yīng)用鋪平道路。 另一方面,用太陽光分解水制氫,為人類提供清潔燃料,被視為化學(xué)的圣杯。光解水的核心在于尋找能夠高效吸收太陽光的半導(dǎo)體催化劑�？上У氖�,傳統(tǒng)的金屬氧化物催化劑帶隙較大,僅能夠吸收太陽光的紫外部分,而紫外光僅占太陽光能量的7%左右,導(dǎo)致太陽光利用效率很低。其它一些金屬化合物雖然能夠吸收可見光,但是本身穩(wěn)定性太差,容易分解,或者催化活性低,量子產(chǎn)率難以滿足實際要求。為此,我們通過第一性原理計算探索能夠有效利用可見光,甚至紅外光,并且穩(wěn)定性良好的半導(dǎo)體催化劑,為實現(xiàn)高效光解水制氫指明一條方向。 本論文共分為三章。第一章介紹材料設(shè)計的理論基礎(chǔ),即計算量子化學(xué)。根據(jù)基本變量選擇的不同,量子化學(xué)可分為兩種不同的表達途徑：從波函數(shù)出發(fā)和從電子密度出發(fā)�；诓ê瘮�(shù)的量子化學(xué)的優(yōu)點是精度可以達到很高,缺點就是費時。依目前的計算機硬件條件,僅適合處理幾個到幾十個原子的分子或團簇體系。而且對化學(xué)家來說,波函數(shù)距離直觀感覺太遠：它是3N維函數(shù)(N為電子數(shù)),難以想象它的具體形狀。相反地,電子密度只是三維空間函數(shù),是一個很直觀的量。因此,基于電子密度的量子化學(xué),即密度泛函理論,得到化學(xué)家的偏愛。同時,引入電子密度作為基本變量后,薛定諤方程更加容易求解,因此計算速度很快而且結(jié)果的精度基本能達到化學(xué)家的要求,這就使得處理上百甚至上千個原子的體系成為可能。故而,密度泛函理論被廣泛地應(yīng)用于大體系和固態(tài)周期體系的模擬。本論文的工作就是基于密度泛函理論,對材料的性質(zhì)進行計算和設(shè)計。在本章中,我們較為詳細地介紹了密度泛函理論的起源和發(fā)展,以及具體的理論框架。 第二章中我們介紹自旋電子學(xué)材料的理論設(shè)計。首先,為解決在自旋電子學(xué)中如何用電場調(diào)控載流子自旋取向這一關(guān)鍵性的科學(xué)問題,我們在概念上提出了一種新型的自旋電子學(xué)材料,即雙極磁性半導(dǎo)體(英文全稱為bipolar magnetic semiconductors,縮寫為BMS)。此類材料具有特殊的能帶構(gòu)造,通過它的電流不僅可以達到完全的自旋極化,而且載流子的自旋取向可以簡單地通過加門電壓的方法直接進行調(diào)制�；谶@一概念,我們相繼設(shè)計了多種雙極磁性半導(dǎo)體體系,包括一維化學(xué)修飾的碳納米管,準二維La(Mn,Zn)AsO合金,MnPSe3納米片,表層摻雜Sic納米薄膜,三維四元Heusler合金FeVXSi (X=Ti, Zr)等。其次,為解決磁性半導(dǎo)體的磁有序溫度普遍低于室溫這一難題,我們在概念上提出了一種新的解決方案,即非對稱反鐵磁半導(dǎo)體(英文全稱為asymmetric antiferromagnetic semiconductors,縮寫為AAMS)。在這類半導(dǎo)體中,磁矩來源于不同的磁性原子,各相鄰磁矩以反鐵磁排列耦合在一起。由于很強的反鐵磁超交換作用,磁耦合溫度很容易超過室溫。同時,半導(dǎo)體的價帶和導(dǎo)帶是高度自旋極化的,這是不同磁性原子之間磁軌道能級相互交錯所導(dǎo)致的。因此非對稱反鐵磁半導(dǎo)體同時具備室溫磁序以及大自旋極化兩個性質(zhì)�；谶@一設(shè)計想法,我們在鈣鈦礦型A2CrMO6(A=Ca,Sr,Ba; M=Ru, Os)系列體系實現(xiàn)了該類材料,從而驗證了所提概念的可行性。再次,我們設(shè)計了一種新型二維鐵磁半導(dǎo)體,即CrXTe3(X=Si, Ge)納米片,其價帶和導(dǎo)帶具有相同的自旋極化方向,能夠用作納米尺度的自旋產(chǎn)生源。最后,我們介紹對另一類自旋電子學(xué)材料,即半金屬磁性材料的設(shè)計工作。為了構(gòu)造能在常溫下工作的自旋電子器件,半金屬必須具有高于室溫的鐵磁居里溫度,較寬的半金屬能隙,以及顯著的磁各向異性能。不幸的是,人們至今還沒有找到同時滿足這些條件的材料�；谇懊嫣岢龅呐c“1111”型鐵基超導(dǎo)體同構(gòu)的層狀La(Mn,Zn)AsO合金,我們對其進行元素替代摻雜,使該材料從反鐵磁半導(dǎo)體轉(zhuǎn)變成鐵磁半金屬。理論預(yù)測半金屬的居里溫度高于室溫,半金屬能隙高達0.74eV。同時,體系內(nèi)稟的準二維結(jié)構(gòu)賦予了半金屬材料極高的磁各向異性能,其理論預(yù)測值比目前已獲得的半金屬材料高一至兩個數(shù)量級。 在第三章,我們介紹光解水催化劑的理論設(shè)計。一方面,基于單層氮化硼納米片,我們通過化學(xué)修飾的方法設(shè)計了一種新型非金屬光解水催化劑,它能夠有效地吸收可見光催化分解水制氫。另一方面,我們提出了一種光解水制氫的新機制,可以把太陽光的紅外段也有效地利用起來。在傳統(tǒng)理論中,光催化劑的能隙至少要大于水分解反應(yīng)吸熱(1.23eV),因而占太陽光能量近一半的紅外光無法被吸收用來分解水制氫。我們首次提出利用具有內(nèi)稟電偶極矩的二維納米催化劑,可突破傳統(tǒng)理論對催化劑能隙的限制。這種催化劑存在偶極內(nèi)電場,吸附在催化劑兩個表面上的水分子會感受到不同的靜電勢,從而導(dǎo)致兩個表面上水的氧化還原電勢變得不再相同。如果氧化和還原分別發(fā)生在不同的表面,催化劑受到的能隙限制原則上將不再存在。在這一新的光解水機制中,不僅紫外光和可見光,紅外光也可以用來促使水分解產(chǎn)生氫氣。另外,這種催化劑的光激發(fā)是一個電荷轉(zhuǎn)移過程,電子和空穴分別產(chǎn)生在兩個不同的表面,催化劑固有偶極電場有效地促進了光生電子空穴對的空間分離,并做功幫助水分解產(chǎn)生氫氣。基于這一機制,我們設(shè)計了一種雙層氮化硼納米體系,其兩個表面分別用氫和氟修飾。理論計算與模擬表明這是一種有效的紅外光催化分解水體系。
[Abstract]:Material is closely related to people's life and promotes the development of human society. Every major economic and social change in history can almost find the shadow of material. For example, the discovery and large-scale application of semiconductors have led to the human society entering the microelectronic age, and so far it has benefited. However, the nature has existed. The limited material has been unable to meet the growing material needs of people. This requires people to design new materials with specific functions, based on existing materials, using various physical or chemical means. In the view of the experimenter, this is a constantly trying and arduous task that needs to be put in a lot of time in this process. And energy will inevitably lead to waste of experimental resources.
The development of computational quantum chemistry has brought the gospel to this dilemma. Using the first principle to predict the properties of the material, the preliminary screening of the material according to the nature of the prediction, and then the experimental verification, will greatly improve the efficiency of the experimenter's work and shorten the cycle of material design. The purpose of this paper is to introduce the purpose of this paper. Based on the first principle, we work on the functional design of spintronic materials and photolysis of water catalyzed materials. These two areas seem unrelated, and are actually caused by the increasingly serious energy and environmental crisis. The former is aimed at getting higher speed with lower energy, and the latter looking for cleanliness. The sustainable energy sources, that is, solar energy, is the substitute of fossil energy.
Spintronics has become a hot spot in recent years. However, spintronics is facing three major challenges: spin generation and injection. The long range transport of spin and the regulation and detection of spin will depend mainly on the search for spintronic materials with specific properties, such as magnetic semiconductors, semi metal materials, etc. Although many spintronics materials have been proposed in succession, they are still larger than the actual applications. The reasons include the spin heat turnover resulting in the destruction of semi metal, the magnetic ordering temperature below room temperature, the synthesis difficulty or the difficult control. We focus on the design of new spintronics materials with special functions through the first principle, and the search for spintronic materials available in the room temperature ring, for spin electricity. The synthesis and application of the subsystems are paved.
On the other hand, using sunlight to break water to produce hydrogen and provide clean fuel for human beings, it is regarded as the Holy Grail of chemistry. The core of the photolysis of water is to find a semiconductor catalyst that can absorb the sun's light efficiently. Unfortunately, the traditional metal oxide catalyst has a large band gap and can only absorb the ultraviolet part of the sun light, while the ultraviolet light only accounts for the sun's light. About 7% of the energy, resulting in low solar efficiency. Although some other metal compounds can absorb visible light, but their stability is too poor, easy to decompose, or low catalytic activity, the quantum yield is difficult to meet the actual requirements. Therefore, we can use the first principle to explore the effective use of visible light, even infrared light. Moreover, the semiconductor catalyst with good stability indicates a direction for the production of hydrogen by efficient photolysis of water.
This paper is divided into three chapters. The first chapter introduces the theoretical basis of material design, which is the calculation of quantum chemistry. According to the selection of basic variables, quantum chemistry can be divided into two different ways of expression: starting from the wave function and starting from the electron density. The advantage of the quantum chemistry based on the wave function is that the precision can be high and the disadvantage is that Time consuming. According to the current computer hardware conditions, it is only suitable for dealing with several molecules or clusters of several dozens of atoms. And for chemists, the distance of the wave function is far too far away: it is a 3N dimensional function (N is an electron number), and it is difficult to imagine its specific shape. On the contrary, the electrical subdensity is only a three-dimensional space function, and it is a very intuitive one. Therefore, the electron density based quantum chemistry, the density functional theory, gets the preference of the chemist. At the same time, after introducing the electron density as the basic variable, the Schrodinger equation is easier to solve, so the calculation speed is very fast and the accuracy of the result can reach the requirement of the chemist, which makes the processing hundreds of thousands of atoms. Therefore, the density functional theory is widely used in the simulation of large systems and solid state periodic systems. The work of this thesis is based on the density functional theory and the calculation and design of the properties of the material. In this chapter, we introduce the origin and development of the density function theory, as well as the specific theory in this chapter. Frame.
In the second chapter, we introduce the theoretical design of spintronics materials. First, in order to solve the key scientific problem of how to use electric field to regulate the carrier spin orientation in spintronics, we have proposed a new kind of spintronic material, that is, bipolar magnetic semiconductor (the full name of bipolar magnetic semicon in English. Ductors, abbreviated as BMS). This kind of material has a special band structure, which can not only achieve full spin polarization through its current, but also the spin orientation of the carrier can be directly modulated by the method of adding the gate voltage. Based on this concept, we have set up a variety of bipolar magnetic semiconductor systems, including one dimension. Chemically modified carbon nanotubes, quasi two-dimensional La (Mn, Zn) AsO alloy, MnPSe3 nanoscale, surface doped Sic nanometers, three dimensional four element Heusler alloy FeVXSi (X=Ti, Zr), etc. Secondly, to solve the problem that magnetic ordered temperature of magnetic semiconductors is generally lower than room temperature, we have proposed a new solution, that is, asymmetric antiferromagnetism. The semiconductor (in English is called asymmetric antiferromagnetic semiconductors, abbreviated as AAMS). In this type of semiconductors, the magnetic moments are derived from different magnetic atoms, and the adjacent magnetic moments are coupled in the antiferromagnetic arrangement. The magnetic coupling temperature is easy to exceed the room temperature because of the strong antiferromagnetic exchange. The conduction band is highly spin polarized, which is caused by the interlacing of the magnetic orbital energy levels between different magnetic atoms. Therefore, the asymmetric antiferromagnetic semiconductor has two properties at the same time at room temperature magnetic order and large spin polarization. Based on this design idea, we have implemented this kind of A2CrMO6 (A= Ca, Sr, Ba; M=Ru, Os) system. Again, we have designed a new two-dimensional ferromagnetic semiconductor, CrXTe3 (X=Si, Ge) nanoscale. The valence band and the guide band have the same spin polarization direction and can be used as the nanoscale spintronic source. Finally, we introduce to another kind of spintronics material, the semi metal magnetic material. Design work. In order to build a spintronic device capable of working at room temperature, semi metal must have a ferromagnetic Curie temperature above room temperature, a wider half metal gap, and significant magnetic anisotropy. Unfortunately, people have not yet found the material to meet these conditions at the same time. Based on the previous and "1111" type A layered La (Mn, Zn) AsO alloy with an isomorphic iron base superconductor is doped to make the material change from antiferromagnetic semiconductor to ferromagnetic semi metal. It is predicted that the Curie temperature of semi metal is higher than room temperature, the half metal gap is as high as 0.74eV., and the intrinsic quasi two-dimensional structure endows semi metal materials with very high magnetic properties. Its theoretical prediction value is about one to two orders of magnitude higher than that of half metallic materials currently obtained.
In the third chapter, we introduce the theoretical design of the photodissociation water catalyst. On the one hand, based on the monolayer boron nitride nanoscale, we designed a novel non-metallic photodissociation water catalyst by chemically modified method. It can effectively absorb the visible light catalytic decomposition of water for hydrogen production. On the other hand, we have proposed a new mechanism for the photodissociation of hydrogen. In the traditional theory, the energy gap of the photocatalyst is at least greater than that of the water decomposition reaction (1.23eV), so that the infrared light, which accounts for nearly half of the solar energy, can not be absorbed into the decomposition of water for hydrogen production. It breaks through the limitation of the traditional theory on the energy gap of the catalyst. This catalyst has the dipole internal electric field, and the water molecules adsorbed on the two surface of the catalyst will feel the different electrostatic potential, resulting in the redox potential of the water on the two surfaces. The energy gap limit will not exist in principle. In this new photolysis mechanism, not only ultraviolet and visible light, but also infrared light can also be used to induce water to produce hydrogen. In addition, the light excitation of the catalyst is a charge transfer process, and the electrons and holes are produced on two different surfaces, and the intrinsic dipole electric field of the catalyst is effective. Based on this mechanism, we designed a two-layer boron nitride nano system, and the two surfaces were modified with hydrogen and fluorine respectively. The theoretical calculation and simulation show that this is an effective infrared photocatalytic decomposition water system.
【學(xué)位授予單位】：中國科學(xué)技術(shù)大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TB39;O643.36

【參考文獻】

相關(guān)期刊論文 前1條

1 CHEN Peng;ZHANG GuangYu;;Carbon-based spintronics[J];Science China(Physics,Mechanics & Astronomy);2013年01期

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本文編號：2146167