In situ high resolution transmission electron microscopy investigation of deformation mechanism in sub-10-nm Au crystals

Abstract

In situ high resolution transmission electron microscopy investigations were performed on sub-10-nm Au crystals. The effects of tensile loading direction and crystal size on the deformation mechanism of Au crystals were analysed. For the Au crystals with a width below 2 nm, the surface atom diffusion with a phenomenon of layer by layer peeling is the main deformation mechanism and the tensile loading direction plays negligible effect. For the Au crystals with a width over 7 nm, the dislocations generated form surface and gliding into crystal dominate the plastic deformation and the tensile loading direction plays important role. Lomer dislocations are produced and destructed by dislocation reaction during tensile strain process in <001> oriented Au crystal. The Schmid law is the key intrinsic issue controlling the deformation mechanism for the nanowires with a size larger than 7 nm.     

Microstructure and Surface Topography Evolution of Ti and Ni Thin Structures

This paper presents the results of a study of microstructure and surface topography of electron-beam deposited nano-scale (10-500 nm thick) thin films of Ti and Ni. The films are deposited on substrates with different moduli (Ni, NaCl, and Si). The microstructure of each film is characterized using transmission electron microscopy. The surface topography is also studied using atomic force microscopy. The microscopic observations show that the grain size and film roughness increase with increasing film thickness. The grain coarsening observed in Ni films was greater than that observed in Ti films. Furthermore, the effects of coarsening amplitude on (001) oriented NaCl crystals were much greater than those on stiffer (001) oriented silicon substrates. The implications of the results are then discussed for applications of thin films in biomedical engineering, microelectronics, and micro-electro-mechanical systems (MEMS).     

Analysis of ISE in dynamic hardness measurements of β-Sn single crystals using a depth-sensing indentation technique

The dynamic indentation tests on (001) plane β-Sn single crystals having different growth directions under different peak indentation test load (10, 20, 30, 40, and 50 mN) has been investigated. The experimental results reveal that the measured hardness values exhibit a peak-load dependence, i.e., indentation size effect (ISE). Such peak-load dependence is then analyzed using the Meyer's law, the Hays–Kendall's approach, the Elastic/Plastic Deformation model, the Proportional Specimen Resistance (PSR) model, and the Modified PSR (MPSR) model. As a result, Modified PSR model is found to be the most effective for dynamic hardness determination of β-Sn single crystals.     

Study of indentation size effect and microhardness of SrLaAlO<Subscript>4</Subscript> and SrLaGaO<Subscript>4</Subscript> single crystals

The load dependence of the Vickers microhardness of SrLaAlO₄ and SrLaGaO₄ single crystals, using a PMT-3 hardness tester, has been investigated and analysed from the standpoint of various theoretical models. On the (100) and (001) planes of these crystals, reverse indentation size effect was observed. Analysis of the experimental data revealed: (1) the indentation size effect is best described by Meyer's law and the proportional specimen resistance model of Li and Bradt, (2) indentation-induced cracking model for reverse indentation size effect and Meyer's law cannot be used to determine the hardness of the crystals, (3) as shown by the negative values of the load-dependent quantities in Hays-Kendall's approach and Li-Bradt model, the origin of indentation size effect is associated with the processes of relaxation of indentation stresses, and (4) the load-dependent and load-independent quantities of different models are interrelated and are intimately connected with the orientation and chemical composition of the crystals. It was also found that the plots of the ratio of indentation load to indentation diagonal against indentation diagonal for a sample exhibit two different slopes with a transition in the slopes occurring at an indentation diagonal, whose value depends on the indenter orientation, indented plane and chemical composition of the crystals. The physical significance of the appearance of these transitions and the nature of load-independent indentation microhardness are discussed. Electronic Publication     

Microstructural damage analysis of SnAgCu solder joints and an assessment on indentation procedures

In solder joint reliability, solder/pad adjoining interface is crucial, the quality of which is determined by the metallization. In this paper, microstructural analyses of SnAgCu alloy and soldered joints are conducted in direct connection with the metallization. Solder balls, solder paste and cast SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au. Substrate influence on bulk and interfacial metallurgy is examined and a correlation between interfacial microstructure and the corresponding damage paths is established. Damage localizes at the bonding interfaces with a strong influence of intermetallic layers and primary crystals. Crack propagation is studied with Cu and Ni/Au substrates and the cracking mechanism in principal directions is scrutinized. In BGA production, different reflow parameters are investigated, and an optimum bumping procedure is established. Nano-indentation is used for the mechanical characterization of the solder alloy. An assessment on indentation parameters for soft solders is conducted and the influence of Ag content on material properties of SnAgCu is presented.     

Ultramicrohardness experiments on vapour-deposited films of pure metals and alloys

The ultramicrohardness of thin alloy films is determined by indentation experiments at very low loads. The indentation apparatus is briefly described. We measured the hardness of a number of pure metal films, either vapour deposited or magnetron sputtered. In addition, a detailed study was made of the composition dependence of the hardness of the six binary alloy systems of AgAuCu and AuCoNi. These films were prepared by high vacuum vapour deposition. The thickness of all the films was 1 μm.The hardness data were interpreted with the help of the metastable alloy phase diagram, e.g. as obtained by transmission electron microscopy. The results are discussed in terms of the blocking of the motion of dislocations, solid solution hardening, compound formation, grain size reduction due to alloying, and differences in atomic radii.     

Metal positioning on silicon surfaces using the etching of buried dislocation arrays

Large-area Si(001) nanopatterned surfaces obtained by etching dislocation line arrays have been used to drive the positioning of metallic islands. A method combining wafer bonding of (001) silicon on insulator layers and preferential chemical etching allows controlling the periodicity of square trench arrays in the 20-50 nm lateral periodicity range with an accuracy of less than 1 nm and a depth of about 4-5 nm. The interfacial area containing the dislocation line plane can be removed and a single crystal maintaining the morphological patterning can be obtained. It is shown that oxidized or deoxidized silicon nanopatterned surfaces can drive the positioning of Ni, Au and Ag islands for a 20 nm lateral periodicity and that a lateral long range order, directly transferred from the dislocation network, can be obtained in the Ni and Au cases.     

Cadmium oxide whisker crystals grown by the vapour-liquid-solid mechanism using various elements as growth initiators

CdO whisker crystals with micrometre thickness and length of up to 4.5 mm were grown by a simple technology described in previous publications of the authors. The whiskers grow on CdS crystal substrates, previously covered by a thin metal or silicon layer, on annealing in air at atmospheric pressure. The growth initiator used was Ag, Au, AI, Si, Mn, Ni, Pd or Pt. A study was carried out of the effect of impurities on the conditions and efficiency of growth, as well as on the morphology and composition of the whiskers obtained. The small crystals were examined using an optical microscope, a scanning electron microscope, X-ray structural analysis and X-ray microanalysis. The maximum speed of growth obtained with Au was about 1 mm h^–1. The whiskers grow with a face-centred cubic lattice. The largest crystals are in the form of a parallelipiped with facets of {100}. Data are obtained which support the vapour-liquid-solid mechanism of crystal growth. The possibility is also shown of growing whiskers on zinc or cadmium plates covered with a thin layer of Cu, Ag, Au or AI by applying the same technology.     

Some features of indentation creep of LiF single crystals

The dependence of the size of the indentation and dislocation rosettes on loading time was investigated on the (001) plane of LiF single crystals. The measurements were performed in temperature range from room temperature to 170° C. The indentation time was varied from 0.2 to 10³ sec. It was revealed that the change of the indentation size during creep was more significant than the change in dislocation ensemble tracks in the field of the concentrated load. It was shown that the dependence of the length of the dislocation rosette edge arms on loading time, when plotting in double logarithmic scale, was linear. This fact allowed the determination of parameterm, characterizing the dependence of the dislocation velocity on stress, using creep experiments. The values ofm proved to be in good agreement with the results obtained by different methods.     

Nucleation of small metal particles on ultrathin SiO2 films on Si

Metal particles of Pt, Ni, Au and Ag, typically less than 50 Å in diameter, were deposited by vacuum evaporation onto substrates of ultrathin (～25 Å) SiO₂ on Si. The particle size and charge were determined by transmission electron microscopy and MOS type capacitance-voltage measurements, respectively. It was observed that the Pt and Ni particles were smaller and more closely spaced than Au and Ag particles under comparable deposition conditions. The particle charge for Pt and Ni appeared to exceed that for Au and Ag for the same substrate coverage, suggesting possible charge contributions to heterogeneous nucleation. Since Pt and Ni particles were observed to carry a positive charge, they are presumably stabilized at negative substrate sites; the opposite appears to be true for Au and Ag.     

Nano-scaled Pt/Ag/Ni/Au contacts on p-type GaN for low contact resistance and high reflectivity

We synthesized the vertical-structured LED (VLED) using nano-scaled Pt between p-type GaN and Ag-based reflector. The metallization scheme on p-type GaN for high reflectance and low was the nano-scaled Pt/Ag/Ni/Au. Nano-scaled Pt (5 A) on Ag/Ni/Au exhibited reasonably high reflectance of 86.2% at the wavelength of 460 nm due to high transmittance of light through nano-scaled Pt (5 A) onto Ag layer. Ohmic behavior of contact metal, Pt/Ag/Ni/Au, to p-type GaN was achieved using surface treatments of p-type GaN prior to the deposition of contact metals and the specific contact resistance was observed with decreasing Pt thickness of 5 A, resulting in 1.5 x 10(-4) ohms cm2. Forward voltages of Pt (5 A)/Ag/Ni contact to p-type GaN showed 4.19 V with the current injection of 350 mA. Output voltages with various thickness of Pt showed the highest value at the smallest thickness of Pt due to its high transmittance of light onto Ag, leading to high reflectance. Our results propose that nano-scaled Pt/Ag/Ni could act as a promising contact metal to p-type GaN for improving the performance of VLEDs.     

Ferromagnetic resonance in nanomagnetic metal core and noble metal shell systems

The change in the line widths in the ferromagnetic resonance (FMR) spectra of Co and Ni nanoparticles upon shell formation with noble metals like gold or silver are described. The Ni(core)Ag(shell), Co(core)Ag(shell), and CO(core)Au(shell) nanoparticles were prepared by a simple transmetallation reaction between the Co and Ni nanoparticles and the Ag+ or AuCl4- ions. It is revealed that the FMR line width decreases upon Ag shell formation whereas it increases upon core-shell composite formation with Au. Several probable explanations such as the differences in size distributions before and after the reaction or the changes occurring in shape anisotropy of the particles due to the shell formation or the different extents of electronic interaction between the core and shell materials have been offered for this observation.     

Preparation and characterization of gold nanocrystals on NaCl(100) surfaces as labels for the dynamical x-ray tracking technique

We report on the fabrication of single crystalline gold nanocrystals of sizes between 10 nm and 25 nm. Gold was deposited under ultra high vacuum conditions on a freshly-cleaved NaCl(001) surface by means of the Molecular Beam Epitaxy technique. The preparation parameters substrate temperature and average thickness of the deposited gold were studied systematically. After the preparation, the crystals were characterized by means of different techniques, such as Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), X-Ray Diffractometry (XRD) and Laue Diffractometry.Measurements of the deposited cluster size, as measured by AFM, and the crystal size, as measured by XRD exhibited similar results at substrate temperatures between approx. 200 °C and 250 °C, indicating a high amount of single gold crystals. TEM images of gold crystals, made after the gold crystals had been dissolved in an aqueous 3-[(3-cholamidopropyl)dimethyl-ammonio]propane solution, support these results.Further, an effect of the substrate temperature during the deposition on the orientation of the gold crystals with respect to the NaCl(001) surface was observed. XRD measurements reveal a perpendicular texture of Au(111) parallel to NaCl(001) at approximately 310 °C. The second detected texture:Au(001)||NaCl(001) (in plane) and Au[100]||NaCl[100] (perpendicular) was observed over the complete studied temperature range (180 °C-330 °C).     

Volume variation of Gruneisen parameters offcc transition metals

The volume variation of the Gruneisen parameters of tenfcc transition metals, up to 40% compression, has been studied on the basis of a model approach proposed by Antonovet al. The results are reasonably good for six metals except for Rh, Ag, Au and Ni when compared with available experimental and other theoretical values. The model requires an appropriate modification for Rh, Ag, Au and Ni.     

Theoretical explanation of Ag/Cu and Cu/Ni nanoscale multilayers softening

Relationship between metallic multilayers hardness and monolayer thickness has been investigated and explained for electroplated Ag/Cu and Cu/Ni multilayers using a modified Thomas-Fermi-Dirac electron theory. Experiments reveal that the peak hardness of Ag/Cu multilayers occurs at the monolayer thickness of about 25 nm, while the peak hardness of Cu/Ni multilayers occurs at about 50 nm. Critical monolayer thickness corresponding to the peak hardness is approximated by the grain size limit of stable dislocations in Ag crystals for the Ag/Cu multilayers and in Cu crystals for Cu/Ni multilayers. Grains size limits are calculated based on a modified Thomas-Fermi-Dirac electron theory. Developed relationship between the critical monolayer thickness and the grains size limit helps understand nanoscale metallic multilayers softening.     

几种单晶半导体材料在压痕下的变形与断裂行为比较

采用显微压痕方法研究了Si、Ge、GaAs和InP四种半导体单晶的变形与断裂行为.通过测量[100]取向单晶体面内的显微硬度,裂纹开裂的临界压痕尺寸以及断裂韧性,分析了这四种材料力学性能的面内各向异性行为.结果表明:在压痕载荷的作用下,Si和Ge的塑性变形以剪切断层为主,而GaAs和InP则通过滑移系的开动协调变形.[100]取向的Si、Ge、GaAs和InP四种单晶的面内显微硬度、弹性模量和断裂韧性表现出不同程度的各向异性.裂纹长度与压痕尺寸间的关系表明,与GaAs和InP相比,Si、Ge具有较小的临界压痕尺寸和拟合直线斜率,这一临界压痕尺寸和拟合直线斜率的变化规律分别与材料的硬度和断裂韧性的变化规律一致.     

Growth and crystalline structure of Ag layers on Au(111)

The Ag/Au(111) adsorption system was investigated by Auger electron and directional elastic peak electron spectroscopies (DEPES). The change of the Auger peak heights for Au (NVV transition, 74 eV) and Ag (MNN transition, 360 eV) recorded during the continuous Ag deposition show the formation of the second layer before the completion of the first monolayer. Further evaporation leads to the nucleation of islands on the not-completed second layer. The intensity of elastically backscattered electrons was measured as a function of the incidence angle of the primary electron beam to obtain stereographic DEPES distributions at energies Ep = 1.7 keV and 2.0 keV. About 8 ML of Ag on Au(111) raises intensity contrast of the total map and of the particular peaks associated with crystallographic directions proving well ordered Ag(111) overlayer formation. The quantitative analysis of the contrasts for experimental and theoretical DEPES maps, the latter obtained by using the multiple scattering theory, suggests the coexistence of the reconstructed and nonreconstructed domains on the Au(111) surface.     

Recrystallization on their substrates of Au (001)/NaCl (001) films into Au (111)/NaCl (001) Part I. Electron Microscopy and electron diffraction observations

The electron-gun evaporation technique for the deposition of Au and Ag films onto (001) NaCl leads to a decrease in the epitaxial temperatures of these materials to 75° and 50°C respectively. This decrease has enabled us to use a wide range of temperatures for film recrystallization without destruction of the substrate. The annealing of the films on their substrates leads to a change in their orientation from metal (001)/NaCl (001) to metal (111)/NaCl (001). In the present work the experimental and kinetic conditions of this transformation process are given.     

Quasi-freestanding,striped WS2 monolayer with an invariable band gap on Au(001)

Revealing the structural/electronic features and interfacial interactions of monolayer MoS2 and WS2 on metals is essential to evaluating the performance of related devices.In this study,we focused on the atomic-scale features of monolayer WS2 on Au(001) synthesized via chemical vapor deposition.Scanning tunneling microscopy and spectroscopy reveal that the WS2/Au(001) system exhibits a striped superstructure similar to that of MoS2/Au(001) but weaker interfacial interactions,as evidenced by experimental and theoretical investigations.Specifically,the WS2/Au(001) band gap exhibits a relatively intrinsic value of ～ 2.0 eV.However,the band gap can gradually decrease to ～ 1.5 eV when the sample annealing temperature increases from ～370 to 720 ℃.In addition,the doping level (or Fermi energy) of monolayer WS2/Au(001) varies little over the valley and ridge regions of the striped patterns because of the homogenous distributions of point defects introduced by annealing.Briefly,this work provides an in-depth investigation into the interfacial interactions and electronic properties of monolayer MX2 on metal substrates.