Local thermal conductivity of polycrystalline AlN ceramics measured by scanning thermal microscopy and complementary scanning electron microscopy techniques

The local thermal conductivity of polycrystalline aluminum nitride (AlN) ceramics is measured and imaged by using a scanning thermal microscope (SThM) and complementary scanning electron microscope (SEM) based techniques at room temperature.The quantitative thermal conductivity for the AlN sample is gained by using a SThM with a spatial resolution of sub-micrometer scale through using the 3ω method.A thermal conductivity of 308 W/m·K within grains corresponding to that of high-purity single crystal AlN is obtained.The slight differences in thermal conduction between the adjacent grains are found to result from crystallographic misorientations,as demonstrated in the electron backscattered diffraction.A much lower thermal conductivity at the grain boundary is due to impurities and defects enriched in these sites,as indicated by energy dispersive X-ray spectroscopy.     

Dopant profiling in silicon nanowires measured by scanning capacitance microscopy

Silicon nanowires (SiNWs) with axial doping junctions were synthesized via the Au‐catalyzed vapor–liquid–solid growth method with the use of HCl. In this work, dopant profiling from three axially doped SiNWs with p–i, p–n and n–i–p junctions were investigated using both scanning electron microscopy (SEM) and scanning capacitance microscopy (SCM). It turns out that observed doping contrasts in SEM are also affected by the surface roughness and sample charging. In contrast, SCM allows us to delineate with sub‐10 nm resolution the electrical junctions and provides a relative value of the doping concentration in each segment of the NW. SCM clearly evidences the expected doping regions within these SiNWs thanks to the addition of HCl during the growth that strongly prevents shell overgrowth. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

高分辨扫描透射电子显微镜原理及其应用

扫描透射电子显微术是目前应用最广泛的电子显微表征手段之一,具有分辨率高、对化学成分敏感和图像直观易解释等特点。其中高分辨扫描电子显微镜可以直接获得原子分辨率的Z 衬度像,结合X射线能谱(EDS)和电子能量损失谱(EELS),可在亚埃尺度上对材料的原子和电子结构进行分析。文章简述了扫描透射电子显微镜的基本原理及其应用现状,重点论述了高角环形暗场(HAADF)和环形明场(ABF)像的成像原理、特征和应用。此外,文中还对原子尺度分辨率的X射线能谱及电子能量损失谱元素分析方法进行了简述。     

二维原子晶体的低电压扫描透射电子显微学研究

二维原子晶体材料,如石墨烯和过渡金属硫族化合物等,具有不同于其块体的独特性能,有望在二维半导体器件中得到广泛应用.晶体中的结构缺陷对材料的物理化学性能有直接的影响,因此研究结构缺陷和局域物性之间的关联是当前二维原子晶体研究中的重要内容,需要高空间分辨率的结构研究手段.由于绝大部分二维原子晶体在高能量高剂量的电子束辐照下容易发生结构损伤,利用电子显微方法对二维原子晶体缺陷的研究面临诸多挑战.低电压球差校正扫描透射电子显微(STEM)技术的发展,一个主要目标就是希望在不损伤结构的前提下对二维原子晶体的本征结构缺陷进行研究.在STEM下,多种不同的信号能够被同步采集,包括原子序数衬度高分辨像和电子能量损失谱等,是表征二维原子晶体缺陷的有力工具,不但能对材料的本征结构进行单原子尺度的成像和能谱分析,还能记录材料结构的动态变化.通过调节电子束加速电压和电子辐照剂量,扫描透射电子显微镜也可以作为电子刻蚀二维原子晶体材料的平台,用于加工新型纳米结构以及探索新型二维原子晶体的原位制备.本综述主要以本课题组在石墨烯和二维过渡金属硫族化合物体系的研究为例,介绍低电压扫描透射电子显微学在二维原子晶体材料研究中的实际应用.     

扫描电子显微镜法测定金属衬底上石墨烯薄膜的覆盖度

利用化学气相沉积法生长在金属衬底上的石墨烯薄膜,由于其尺寸的可控性和转移的便利性,被广泛用作各种透明电极.石墨烯薄膜的方块电阻是衡量其品质的重要指标之一,而石墨烯覆盖完全是保证薄膜拥有优良导电性能的基本前提.本文通过研究评估不确定度的分量,提出利用扫描电子显微镜像素计算微区和宏观覆盖度的方法.考虑到石墨烯薄膜覆盖区域与未覆盖区域边界的确定,以及晶畴数目的选取这两个因素对覆盖度测定造成的误差.通过微区有效扫描电子显微镜图像的确定、宏观石墨烯薄膜有效扫描电子显微镜图像的测量数目以及宏观石墨烯薄膜覆盖均匀性的表达,系统研究了化学气相沉积法生长在金属衬底上的石墨烯薄膜的微区覆盖度、宏观覆盖度和覆盖均匀性.该方法通过获得有限次微区扫描电子显微镜图像,不仅可以计算宏观石墨烯薄膜的覆盖度,还可以给出覆盖均匀性,既节省了测量时间,同时也能保证测量有效性.     

Features of formation of channels during laser treatment of AlN ceramics

Using electron microscopy and X-ray microanalysis, the formation of tracks by laser processing of AlN:Y₂O₃ ceramic surfaces was investigated. The main process of channel formation is the decomposition of aluminum nitride (AlN). Due to aluminothermic reduction of Y₂O₃, the metallization of channel surfaces occurs. The main products of ablation are Al₂O₃, AlN, and AlON.     

Microstructural analysis by scanning thermal microscopy of a nanocrystalline Fe surface induced by ultrasonic shot peening

Experiments were carried out using a scanning thermal microscope (SThM) to investigate the ultrafine-grained microstructures in the surface layer of a pure iron sample produced by ultrasonic shot peening. The analysis of thermal properties by SThM shows that the thermal conductivity strongly depends on the grain size of the microstructure. The different contrasts on the thermal conductivity images are used to estimate the thickness of the affected surface layer produced by ultrasonic shot peening. The results demonstrate that the SThM can be used as a powerful tool for the microstructural analysis of the ultrafine-grained surface layer.     

Abnormal thermal conductivity of [720]‐oriented BaTiO3 single crystal

The 3ω ‐scanning thermal microscopy and piezoresponse force microscopy techniques were combined to characterize the local thermal conductivity (λ) of ferroelastic domains in [720]‐oriented and [001]‐oriented BaTiO₃ single crystal. The [720]‐oriented BaTiO₃ showed a local λ value of 3.39 W/m·K, while it was 5.95 W/m·K for [001]‐oriented BaTiO₃. The underlying mechanism for such drastic thermal conductivity reduction in [720]‐oriented BaTiO₃ is ascribed to a stronger phonon‐scattering effect due to the high strains appearing in [720]‐oriented BaTiO₃ single crystals. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical and thermal transport properties of CdO ceramics

High temperature electrical and thermal transport properties, that is, electrical conductivity, Seebeck coefficient and thermal conductivity, of CdO ceramics have been investigated. Because of the good electrical properties and low thermal conductivity, the dimensionless figure-of-merit ZT of the CdO ceramics reaches 0.34 at 1023 K. This value is comparable to the best reported ZT for the n-type oxide ceramic thermoelectric materials and remains as potential to be further improved by porosity controlling or nanostructuring.     

A correlative optical microscopy and scanning electron microscopy approach to locating nanoparticles in brain tumors

The growing use of nanoparticles in biomedical applications, including cancer diagnosis and treatment, demands the capability to exactly locate them within complex biological systems. In this work a correlative optical and scanning electron microscopy technique was developed to locate and observe multi-modal gold core nanoparticle accumulation in brain tumor models. Entire brain sections from mice containing orthotopic brain tumors injected intravenously with nanoparticles were imaged using both optical microscopy to identify the brain tumor, and scanning electron microscopy to identify the individual nanoparticles. Gold-based nanoparticles were readily identified in the scanning electron microscope using backscattered electron imaging as bright spots against a darker background. This information was then correlated to determine the exact location of the nanoparticles within the brain tissue. The nanoparticles were located only in areas that contained tumor cells, and not in the surrounding healthy brain tissue. This correlative technique provides a powerful method to relate the macro- and micro-scale features visible in light microscopy with the nanoscale features resolvable in scanning electron microscopy.     

Investigation of thermal effects in through-silicon vias using scanning thermal microscopy

Results of quantitative investigations of copper through-silicon vias (TSVs) are presented. The experiments were performed using scanning thermal microscopy (SThM), enabling highly localized imaging of thermal contrast between the copper TSVs and the surrounding material. Both dc and ac active-mode SThM was used and differences between these variants are shown. SThM investigations of TSVs may provide information on copper quality in TSV, as well as may lead to quantitative investigation of thermal boundaries in micro- and nanoelectronic structures. A proposal for heat flow analysis in a TSV, which includes the influence of the boundary region between the TSV and the silicon substrate, is presented; estimation of contact resistance and boundary thermal conductance is also given.     

Contamination mitigation strategies for scanning transmission electron microscopy

Modern scanning transmission electron microscopy (STEM) enables imaging and microanalysis at very high magnification. In the case of aberration-corrected STEM, atomic resolution is readily achieved. However, the electron fluxes used may be up to three orders of magnitude greater than those typically employed in conventional STEM. Since specimen contamination often increases with electron flux, specimen cleanliness is a critical factor in obtaining meaningful data when carrying out high magnification STEM. A range of different specimen cleaning methods have been applied to a variety of specimen types. The contamination rate has been measured quantitatively to assess the effectiveness of cleaning. The methods studied include: baking, cooling, plasma cleaning, beam showering and UV/ozone exposure. Of the methods tested, beam showering is rapid, experimentally convenient and very effective on a wide range of specimens. Oxidative plasma cleaning is also very effective and can be applied to specimens on carbon support films, albeit with some care. For electron beam-sensitive materials, cooling may be the method of choice. In most cases, preliminary removal of the bulk of the contamination by methods such as baking or plasma cleaning, followed by beam showering, where necessary, can result in a contamination-free specimen suitable for extended atomic scale imaging and analysis.     

High thermoelectric performance and low thermal conductivity in K-doped SnSe polycrystalline compounds

SnSe single crystal showed a high thermoelectric zT of 2.6 at 923 K mainly due to an extremely low thermal conductivity 0.23 W m⁻¹ K⁻¹. It has anisotropic crystal structure resulting in deterioration of thermoelectric performance in polycrystalline SnSe, providing a low zT of 0.6 and 0.8 for Ag and Na-doped SnSe, respectively. Here, we presented the thermoelectric properties on the K-doped K_xSn_1−xSe (x = 0, 0.1, 0.3, 0.5, 1.5, and 2.0%) polycrystals, synthesized by a high-temperature melting and hot-press sintering with annealing process. The K-doping in SnSe efficiently enhances the hole carrier concentration without significant degradation of carrier mobility. We find that there exist widespread Se-rich precipitates, inducing strong phonon scattering and thus resulting in a very low thermal conductivity. Due to low thermal conductivity and moderate power factor, the K_0.001Sn_0.999Se sample shows an exceptionally high zT of 1.11 at 823 K which is significantly enhanced value in polycrystalline compounds.     

Carrier density distribution in silicon nanowires investigated by scanning thermal microscopy and Kelvin probe force microscopy

The use of scanning thermal microscopy (SThM) and Kelvin probe force microscopy (KPFM) to investigate silicon nanowires (SiNWs) is presented. SThM allows imaging of temperature distribution at the nanoscale, while KPFM images the potential distribution with AFM-related ultra-high spatial resolution. Both techniques are therefore suitable for imaging the resistance distribution. We show results of experimental examination of dual channel n-type SiNWs with channel width of 100 nm, while the channel was open and current was flowing through the SiNW. To investigate the carrier distribution in the SiNWs we performed SThM and KPFM scans. The SThM results showed non-symmetrical temperature distribution along the SiNWs with temperature maximum shifted towards the contact of higher potential. These results corresponded to those expressed by the distribution of potential gradient along the SiNWs, obtained using the KPFM method. Consequently, non-uniform distribution of resistance was shown, being a result of non-uniform carrier density distribution in the structure and showing the pinch-off effect. Last but not least, the results were also compared with results of finite-element method modeling.     

Adsorption geometry of glycine on Cu（001） determined with low—energy electron diffraction and scanning tunnelling microscopy

Using low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) it has been found that glycine molecules adsorbed on Cu(001) can form but only the (2×4) and c(2×4) superstructures. On the basis of the missing LEED spots of the surface, it has been concluded that: each (2×4) unit cell consists of two molecules, one being the mirror image of the other; the C－C axis of both molecules lies in the mirror plane of the Cu substrate without a significant shift and twist from the plane; and the two O atoms of the carboxylate group of both molecules locate at the same height level without significant buckling. According to these conclusions, a structural model has been proposed for the (2×4) superstructure (a model for the c(2×4) superstructure already exists). We argue that the (2×4) and c(2×4) superstructures must have similar specific surface free energy, that their hydrogen bonds must be of N-H-O_II type, and that their local adsorption geometry must be similar or even the same. The advantage of combining STM with LEED to determine surface structures is clearly demonstrated.     

高定向热解石墨表面局域导电增强现象的扫描探针显微学研究

基于导电型原子力显微镜和扫描隧道显微镜的对比观察,研究高定向热解石墨表面上残留石墨片的导电增强现象.根据样品电阻的测量数据,将这种现象归结为导电针尖与石墨表面的点接触问题,并且发现接触电导和接触点处局域的电子密度成正比,从而确定石墨表面的局域导电增强现象的原因在于残留在石墨表面的石墨片具有较高的电子密度. 关键词： 扫描隧道显微镜 导电原子力显微镜 高定向热解石墨 导电性     

热线法测量材料热导率

讨论在SHT-20型热物性自动测量仪上,用热线法测量材料热导率的原理和方法,并给出了测量结果.     

石墨烯的透射电子显微学研究

石墨烯（Graphene）是近几年迅速发展起来的研究热点材料之一.利用透射电子显微镜（TEM）研究Graphene的结构特征和原子动态过程,是 Graphene研究的重要进展.文章评述了利用透射电子衍射方法对Graphene的层数、堆垛方式、取向和表面形貌等结构特征进行的研究工作,介绍了利用高分辨透射电子显微术在Graphene的表面缺陷、边缘结构及吸附原子等研究领域取得的最新结果.     

扫描力显微术在高聚物薄膜玻璃化转变研究中的应用

简述了高聚物薄膜玻璃化转变的复杂性，并结合文章作者的的一些研究结果介绍了扫描力显微术(SFM)在研究高聚物玻璃化转变中的一些方法，包括观察高聚物薄膜形貌的变化，测量其摩擦力、粘附力和弹性模量等物理量的变化，最后指出SFM是研究高聚物薄膜玻璃化转变的有力工具。     

Physical meaning of conductivity spectra for ZnO ceramics

With the help of broadband dielectric spectroscopy in a wide temperature and frequency range, the conductivity spectra of ZnO polycrystalline ceramics are measured and the direct-current-like (DC-like) conductivity and relaxation polarization conductivity are observed successively along the frequency axis. According to the classical Debye theory and Cole-Cole equation, the physical meanings of the two conductivities are discussed. It is found that the DC-like conductivity corresponds to electron transportation over the Schottky barrier at the grainboundary. The relaxation polarization conductivity corresponds to electronic trap relaxation of intrinsic point defects (zinc interstitial and oxygen vacancy). When in the high frequency region, the relaxation conductivity obeys the universal law with the index n equal to the index α in the Cole-Cole equation as an indictor of disorder degree.