表面预处理对石墨烯上范德瓦耳斯外延生长GaN材料的影响

基于范德瓦耳斯外延生长的氮化镓/石墨烯材料异质生长界面仅靠较弱的范德瓦耳斯力束缚,具有低位错、易剥离等优势,近年来引起了人们的广泛关注.采用NH_3/H_2混合气体对石墨烯表面进行预处理,研究了不同NH_3/H_2比对石墨烯表面形貌、拉曼散射的影响,探讨了石墨烯在NH_3和H_2混合气氛下的表面预处理机制,最后在石墨烯上外延生长了1.6μm厚的GaN薄膜材料.实验结果表明:石墨烯中褶皱处的C原子更容易与气体发生刻蚀反应,刻蚀方向沿着褶皱进行;适当NH_3/H_2比的混合气体对石墨烯进行表面预处理可有效改善石墨烯上GaN材料的晶体质量.本研究提供了一种可有效提高GaN晶体质量的石墨烯表面预处理方法,可为进一步研究二维材料上高质量的GaN外延生长提供参考.     

低维原子/分子晶体材料的可控生长、物性调控和原理性应用

本文介绍了高鸿钧课题组在物理所20年来的部分代表性工作.研究的主要方向为低维纳米功能材料的分子束外延可控制备、生长机制、物性调控及其在未来信息技术中的原理性应用.从材料的可控制备入手,结合第一性原理的理论计算,阐明材料生长机制和结构与物性的关系,进而实现物性调控和原理性应用.主要内容有:1)纳米尺度"海马"分形结构的形成及其生长机制;2)STM分辨率的提高及最高分辨Si(111)-7×7原子图像的获得;3)固体表面上功能分子的吸附、组装及其机制;4)稳定、重复、可逆的纳米尺度电导转变与超高密度信息存储;5)固体表面上单分子自旋态的量子调控及其原理性应用;6)原子尺度上朗德g因子的空间分辨及其空间分布不均匀性的发现;7)晶圆尺寸、高质量、单晶石墨烯的制备及原位硅插层绝缘化;8)几种新型二维原子晶体材料的可控构筑及其物性调控;9)"自然图案化"的新型二维原子晶体材料及其功能化.这些工作为低维量子结构的构造、物性调控及其原理性应用奠定了基础.     

超高真空原子尺度Aux/Si(111)-(7×7)表面吸附的电荷分布测量

原子尺度表面吸附Au原子的物理化学性质对研究纳米器件的制备以及表面催化等起着非常重要的作用.利用调频开尔文探针力显微镜研究了室温下Au在Si(111)-(7×7)表面吸附的电荷分布的特性.首先,利用自制超高真空开尔文探针力显微镜成功得到了原子尺度Au在Si(111)-(7×7)不同吸附位的表面形貌与局域接触电势差(LCPD);其次,通过原子间力谱与电势差分析了Au/Si(111)-(7×7)特定原子位置的原子特性,实现了原子识别;并通过结合差分电荷密度计算解释了Au/Si(111)-(7×7)表面间电荷转移与Au的吸附特性.结果显示,Au原子吸附有单原子和团簇形式.其中,Au团簇以6个原子为一组呈六边形结构吸附于Si(111)-(7×7)的层错半单胞内的3个中心原子位;单个Au原子吸附于非层错半单胞的中心顶戴原子位;同时通过电势差测量得知单个Au原子和Au团簇失去电子呈正电特性.表面差分电荷密度结果显示金在吸附过程中发生电荷转移,失去部分电荷,使得吸附原子位置上的功函数局部减少.在短程力、局域接触势能差和差分电荷密度发生变化的距离范围内,获得了理论和实验之间的合理一致性.     

Si_n团簇/石墨烯(n≤6)结构稳定性和储锂性能的第一性原理计算

目前,硅/碳复合材料是锂离子电池最有潜在应用前景的高容量负极材料之一,硅与碳材料的界面状态是影响其电化学性能的重要因素.本文在作为碳材料结构单元的石墨烯表面构建了 Si_n(n≤6)团簇,采用基于密度泛函理论(DFT)的第一性原理方法研究了 Si_n团簇/石墨烯(Si_n/Gr)的几何构型、结构稳定性和电子性质.结果表明,当Si原子数n≤4时,Si_n团簇优先以平行于石墨烯的二维构型沉积在石墨烯表面,当n ≥ 5时,Si_n团簇优先以三维立体构型沉积在石墨烯表面.随着n的增大,Si_n团簇在石墨烯表面的热力学稳定性显著降低,两者之间的界面结合减弱,并且伴随着Si_n团簇与石墨烯之间的电荷转移也越来越少.同时还研究了Si_n/Gr复合构型的储锂能力,Li原子主要存储在Si_n团簇临近的石墨烯表面和Si_n团簇周围,Si_n团簇与石墨烯复合形成的协同作用增强了 Li原子吸附的热力学稳定性.当n≤4时,存储2个Li原子有利于提高xLiSi_n/Gr体系的热力学稳定性,继续增加Li原子数x会导致稳定性降低;当n≥5时,稳定性随着Li原子数x的增多而逐渐降低.     

二氧化硫在改性BC_3表面的吸附特性研究

采用基于密度泛函理论的第一性原理方法观察修饰不同原子(Mo,Pt,Si)来调控BC_3体系的表面结构和反应活性.研究发现:单个原子吸附在BC_3表面具有不同的稳定位,Pt和Si原子吸附在H1位稳定,而Mo原子吸附在H2位稳定.单个原子在完整结构BC_3表面的扩散势垒较小,而掺杂的原子在单空位缺陷BC_3体系中具有极高的稳定性( 6.5 eV),这些掺杂原子显正电性能够有效地调制BC_3体系的电子结构、磁性以及影响二氧化硫(SO_2)的吸附强弱.与Pt原子掺杂的BC_3体系(Pt-BC_3)相比,单个SO_2分子在Si-和Mo-BC_3体系上的吸附能较大,表现出较高的灵敏特性.该研究为设计类石墨烯基功能纳米材料提供参考.     

SiO2/Si衬底上石墨烯的制备与结构表征

在分子束外延(MBE)设备中,利用直接沉积C原子的方法在覆盖有SiO₂的Si衬底(SiO₂/Si)上生长石墨烯,并通过Raman光谱和近边X射线吸收精细结构谱等实验技术对不同衬底温度(500℃,600℃,700℃,900℃,1100℃,1200℃)生长的薄膜进行结构表征.实验结果表明,在衬底温度较低时生长的薄膜是无定形碳,在衬底温度高于700℃时薄膜具有石墨烯的特征,而且石墨烯的结晶质量随着衬底温度的升高而改善,但过高的衬底温度会使石墨烯质量降低.衬底温度为1100℃时结晶质量最好.衬底温度较低时C原子活性较低,难以形成有序的C-sp²六方环.而衬底温度过高时(1200℃),衬底表面部分SiO₂分解,C原子与表面的Si原子或者O原子结合而阻止石墨烯的形成,并产生表面缺陷导致石墨烯结晶变差.     

化学气相沉积法制备的石墨烯晶畴的氢气刻蚀

利用化学气相沉积法在抛光铜衬底上制备出六角形石墨烯晶畴, 并且在高温条件下对石墨烯晶畴进行氢气刻蚀, 利用光学显微镜和扫描电子显微镜对石墨烯晶畴进行观测, 发现高温条件下石墨烯晶畴表面能够被氢气刻蚀出网络状和线状结构的刻蚀条纹. 通过电子背散射衍射测试证明了刻蚀条纹的形态、密度与铜衬底的晶向有密切关系. 通过对比实验证明了石墨烯表面上的刻蚀条纹是由于石墨烯和铜衬底的热膨胀系数不同, 在降温过程中, 石墨烯表面形成了褶皱, 褶皱在高温氢气气氛下发生氢化反应形成的. 对转移到二氧化硅衬底的石墨烯晶畴进行原子力显微镜测试, 测试结果表明刻蚀条纹的形貌、密度与石墨烯表面褶皱的形貌、密度十分相似. 进一步证明了刻蚀条纹是由于褶皱结构被氢气刻蚀引起的. 实验结果表明, 即使在六角形石墨烯晶畴表面也存在褶皱和点缺陷. 本文提供了一种便捷的方法来观察铜衬底上石墨烯褶皱的分布与形态; 同时, 为进一步提高化学气相沉积法制备石墨烯的质量提供了更多参考.     

Cu 表面性质的第一性原理分析

采用第一性原理赝势平面波方法, 计算并详细分析了面心立方Cu晶体及其 (100), (110) 和 (111) 这3个低指数表面的原子结构、 表面能量及表面电子态密度. 表面能的计算结果表明, Cu (111) 表面的结构稳定性最好, Cu (100) 表面次之, Cu (110)表面的结构稳定性最差. 3个表面的表面原子弛豫量随着层数的增加而逐渐减弱. Cu (110) 表面的最表层原子相对收缩最大, Cu (100)表面次之, Cu (111) 表面的最表层原子相对收缩最小. 表面原子弛豫不仅引起表面几何结构的变化, 而且使表面层原子的电子态密度峰形相对晶体内部发生变化, 这是表面能产生的主要原因, 而Cu (110)表面相对于Cu (100)与Cu (111)表面具有高表面活性的主要原因则源于其表面层原子电子态密度在高能级处的波峰相对晶体内部显著的升高. 关键词： Cu 晶体 表面结构 表面能 态密度     

Si面4H-SiC衬底上外延石墨烯近平衡态制备

SiC热解法是制备大面积、高质量石墨烯的理想选择之一.外延石墨烯的晶体质量仍是制约其应用的关键因素之一.本文通过SiC热解法在4H-SiC(0001)衬底上制备单层外延石墨烯.通过引入氩气惰性气氛和硅蒸气,使SiC衬底表面的Si原子升华与返回概率接近平衡,外延石墨烯生长速率大大减慢,单层石墨烯的生长时间从15 min延长至75 min.测试分析表明,生长速率减慢,外延石墨烯中缺陷减少,晶体质量提高,使得外延石墨烯的电性能都得到改善,单层外延石墨烯的最高载流子迁移率达到1200 cm2/V·s,方阻604?/.以上结果表明,控制生长气氛,减慢生长速率是实现高质量外延石墨烯的可行途径之一.     

超高真空条件下分子束外延生长的单层二维原子晶体材料的研究进展

二维原子晶体材料具有与石墨烯相似的晶格结构和物理性质,为纳米尺度器件的科学研究提供了广阔的平台.研究这些二维原子晶体材料,一方面有望弥补石墨烯零能隙的不足;另一方面继续发掘它们的特殊性质,有望拓宽二维原子晶体材料的应用领域.本文综述了近几年在超高真空条件下利用分子束外延生长技术制备的各种类石墨烯单层二维原子晶体材料,其中包括单元素二维原子晶体材料(硅烯、锗烯、锡烯、硼烯、铪烯、磷烯、锑烯、铋烯)和双元素二维原子晶体材料(六方氮化硼、过渡金属二硫化物、硒化铜、碲化银等).通过扫描隧道显微镜、低能电子衍射等实验手段并结合第一性原理计算,对二维原子晶体材料的原子结构、能带结构、电学特性等方面进行了介绍.这些二维原子晶体材料所展现出的优异的物理特性,使其在未来电学器件方面具有广阔的应用前景.最后总结了单层二维原子晶体材料领域可能面临的问题,同时对二维原子晶体材料的研究方向进行了展望.     

Molecular dynamics study of temperature-dependent ripples in monolayer and bilayer graphene on 6H—SiC surfaces

Using classical molecular dynamics and a simulated annealing technique,we show that microscopic corrugations occur in monolayer and bilayer graphene on 6H-SiC substrates.From an analysis of the atomic configurations,two types of microscopic corrugations are identified,namely periodic ripples at room temperature and random ripples at high temperature.Two different kinds of ripple morphologies,each with a periodic structure,occur in the monolayer graphene due to the existence of a coincidence lattice between graphene and the SiC terminated surface(Si-or C-terminated surface).The effect of temperature on microscopic ripple morphology is shown through analysing the roughness of the graphene.A temperature-dependent multiple bonding conjugation is also shown by the broad distribution of the carbon-carbon bond length and the bond angle in the rippled graphene on the SiC surface.These results provide atomic-level information about the rippled graphene layers on the two polar faces of the 6H-SiC substrate,which is useful not only for a better understanding of the stability and structural properties of graphene,but also for the study of the electronic properties of graphene-based devices.     

Nucleation and growth of Si on Pb monolayer covered Si(111) surfaces

With a scanning tunneling microscope (STM), we study the initial stage of nucleation and growth of Si on Pb monolayer covered Si(111) surfaces. The Pb monolayer can work as a good surfactant for growth of smooth Si thin films on the Si(111) substrate. We have found that nucleation of two-dimensional (2D) Pb-covered Si islands occurs only when the substrate temperature is high enough and the Si deposition coverage is above a certain coverage. At low deposition coverages or low substrate temperatures, deposited Si atoms tend to self-assemble into a certain type of Si atomic wires, which are immobile and stable against annealing to ~ 200 °C. The Si atomic wires always appear as a double bright-line structure with a separation of ~ 9 Å between the two lines. After annealing to ~ 200 °C for a period of time, some sections of Si atomic wires may decompose, meanwhile the existing 2D Pb-covered Si islands grow laterally in size. The self-assembly of Si atomic wires indicate that single Si adatoms are mobile at the Pb-covered Si(111) surface even at room temperature. Further study of this system may reveal the detailed atomic mechanism in surfactant-mediated epitaxy.     

Dependence of the atomic structure and surface relief of platinum foils on the annealing and rolling conditions

The effect of cold rolling, polishing, and thermal annealing conditions on the atomic structure and surface geometry of platinum foils has been studied. The surface morphology has been analyzed using low-energy electron diffraction, atomic force microscopy, and scanning tunneling microscopy. The chemical composition of the surface has been evaluated by Auger electron spectroscopy. It has been demonstrated that a variation in the conditions used for the preparation of the samples makes it possible to produce surfaces with different degrees of perfection from atomically smooth to rippled, fractal, and diffraction-disordered surfaces.     

Role of metal ions in growth and stability of Langmuir-Blodgett films on homogeneous and heterogeneous surfaces

Structure and stability of cadmium arachidate (CdA) Langmuir-Blodgett (LB) films on homogeneous (i.e., OH-, H-passivated Si(001) substrates) and heterogeneous (i.e., Br-passivated Si(001) substrates) surfaces were studied using X-ray reflectivity and atomic force microscopy techniques and compared with those of nickel arachidate (NiA) LB films. While on OH-passivated Si, an asymmetric monolayer (AML) structure starts to grow, on H-passivated Si, a symmetric monolayer (SML) of CdA forms, although for both the films, pinhole-type defects are present as usual. However, on heterogeneous Br-passivated Si substrates, a combination of AML, SML, shifted SML and SML on top of AML (i.e., AML/SML), all types of structures are found to grow in such a way that, due to the variation of heights in the out-of-plane direction, ring-shaped in-plane nanopatterns of CdA molecules are generated. Probably due to stronger head-head interactions and higher metal ion-carboxylic ligand bond strength for CdA molecules compared to NiA, easy flipping of SML on top of another preformed SML, i.e. a SML/SML structure formation was not possible and as a result a wave-like modulation is observed for the CdA film on such heterogeneous substrate. The presence of hydrophilic/hydrophobic interfacial stress on the heterogeneous substrate thus modifies the deposited molecular structure so that the top surface morphology for a CdA film is similar to monolayer buckling while that for NiA film is similar to monolayer collapse.     

Epitaxial growth of a graphene single crystal on the Ni(111) surface

The thermally controlled synthesis of graphene from propylene molecules on the Ni(111) surface in ultrahigh vacuum is studied by scanning tunneling microscopy and density functional theory. It is established that the adsorption of propylene on Ni(111) atomic terraces at room temperature results in the dehydration of propylene molecules with the formation of single-atomic carbon chains and in the complete dissociation of propylene at the edges of atomic steps with the subsequent diffusion of carbon atoms below the surface. The annealing of such a sample at 500°С leads to the formation of multilayer graphene islands both from surface atomic chains and by the segregation of carbon atoms collected in the upper nickel atomic layers. The process of formation of an epitaxial graphene monolayer until the complete filling of the nickel surface is controllably observed. Atomic defects seen on the graphene surface are interpreted as individual nickel atoms incorporated into graphene mono- or bivacancies.     

Pulsed laser deposition of Co and growth of CoSi2 on Si(111)

Monolayers of Co were prepared by pulsed laser deposition (PLD) and thermal deposition (TD) on Si(111) substrates. The surface structure and morphology were studied as a function of annealing temperature with scanning tunneling microscopy and low-energy electron diffraction (LEED). Comparing PLD to TD in the monolayer regime, we find surface phases with lower Co content in the top layer for annealing temperatures below 500 °C, indicating an implantation of Co into the Si substrate. The implanted Co leads to an increased intermixing of Co and Si and a higher density of nucleation centers for Co multilayers produced by PLD compared to TD. Multilayer PLD films therefore show an improved film quality, which is detected by an increased intensity of the LEED reflectivity. PACS 68.37.Ef; 61.14.Hg; 68.55.-a     

Identification of Ge/Si intermixing processes at the Bi/Ge/Si(111) surface

The chemical contrast between Si and Ge obtained by scanning tunneling microscopy on Bi-covered Si(111) surfaces is used as a tool to identify two vertical Ge/Si intermixing processes. During annealing of an initially pure Ge monolayer on Si, the intermixing is confined to the first two layers approaching a 50% Ge concentration in each layer. During epitaxial growth, a growth front induced intermixing process acting at step edges is observed. Because of the open atomic structure at the step edges, relative to the terraces, a lower activation barrier for intermixing at the step edge, compared to the terrace, is observed.     

Formation, electronic structure, and stability of film nanophases of transition metals on silicon

This paper is devoted to the study of the morphology, growth, electronic structure, and stability of ultrathin (0.03–3 nm) Co and Fe films on the Si(111) and Si(100) surfaces using Auger-electron spectroscopy, electron-energy loss spectrometry, low-energy electron diffraction, and atomic-force microscopy. It is shown that layer-by-layer growth of the metal with the formation of the film nanophase and the segregation of a submonolayer amount of Si on the film’s nanophase surface occurs during the process of layer-by layer growth of Co and Fe on Si(111)-7 × 7 and Si(100)-2 × 1 at room temperature after the growth of two-dimensional metal phases (the surface phase, the monolayer, and two metal monolayers). After these stages, the formation and growth of the bulk’s metal phase with the dissolution of silicon segregated before occur. It is shown that the upper layers of Si adjoining the surface phase, the monolayer, and two Co and Fe monolayers have respectively three different densities of the electron plasma that are higher than the density of the electron plasma in the volume of the silicon substrate. The nonmonotonous character of the morphological and chemical stability of Fe films with quantum-size thicknesses on Si(100) is discovered. After annealing, the film is first smooth, then it is nonuniform across its thickness; afterwards it is again smooth and then nonuniform across its thickness. In this case, the metal phase, different Fe silicides, and the bulk’s metal phase form successively in Fe films on Si(100) after annealing.     

Flattening of micro-structured Si surfaces by hydrogen annealing

We report atomic scale flattening of surfaces of microstructures formed on Si wafers by furnace annealing. To avoid thermal deformation of the fabricated structures, advantage was taken of hydrogen annealing, which enables us to decrease the relaxation rate of Si surfaces due to surface hydrogenation. We examined cross-sectional shape and sidewall morphology of 3 μm deep trenches on Si(0 0 1) substrates after annealing at 1000 °C under various H₂ pressures of 40-760 Torr. We successfully formed Si trenches with flat surfaces composed of terraces and steps while preserving the designed trench profile by increasing H₂ pressure to 760 Torr.     

6H-SiC(0001)表面graphene逐层生长的分子动力学研究

利用经典分子动力学方法和模拟退火技术分析研究了6H-ＳiC（0001）表面graphene的逐层生长过程及其形貌结构特点．研究表明,经过高温蒸发表面硅原子后,6H-ＳiC（0001）表面的碳原子能够通过自组织过程生成稳定的局部单原子层graphene结构．这种过程类似于6H-ＳiC（0001）表面graphene的形成,其生长和结构形貌演化主要取决于退火温度和表面碳原子的覆盖程度． 研究发现,当退火温度高于1400K时,6H-ＳiC（0001）表面碳原子能形成局部的单原子层graphene结构．这一转变温 关键词： graphene 碳化硅 分子动力学