Profiles with microscopic resolution by single-sided NMR

A single-sided NMR sensor to produce depth profiles with microscopic spatial resolution is presented. It uses a novel permanent magnet geometry that generates a highly flat sensitive volume parallel to the scanner surface. By repositioning the sensitive slice across the object one-dimensional profiles of the sample structure can be produced with a space resolution better than 5 microm. The open geometry of the sensor results in a powerful testing tool to characterize arbitrarily sized objects in a non-destructive way.     

Transport at surface nanostructures measured by four-tip STM

For in situ measurements of local electrical conductivity of well-defined crystal surfaces in ultrahigh vacuum, we have developed two kinds of microscopic four-point probe methods. One is a ‘four-tip STM prober’, in which independently driven four tips of scanning tunneling microscope (STM) are used for four-point probe conductivity measurements. The probe spacing can be changed from 500 nm to 1 mm. The other one is monolithic micro-four-point probes, fabricated on silicon chips, whose probe spacing is fixed around several μm. These probes were installed in scanning-electron-microscopy/electron-diffraction chambers, in which the structures of sample surfaces and probe positions were in situ observed. The probes can be positioned precisely on aimed areas on the sample with aid of piezo-actuators. With these machines, the surface sensitivity in conductivity measurements has been greatly enhanced compared with macroscopic four-point probe method. Then the conduction through the topmost atomic layers (surface-state conductivity) and influence of atomic steps upon conductivity could be directly measured. The STM prober is mainly described here.     

扫描近场光学显微镜的光耦合偶极子模型

为了研究扫描近场光学显微镜中探针和粗糙样品表面的耦合相互作用,提出了一种光耦合偶极子模型.在该模型中,探针和样品突起都由光极化偶极子表示,在准静态电磁场近似的情况下样品表面的诱导极化效应由影像偶极子表示,应用偶极子辐射理论可以得到系统的自洽场方程.此模型提供了一种直观分析扫描近场光学显微镜中探针和样品相互作用机理的方法.在此基础上,进一步讨论了金属样品的近场成像特点和其特有的局域光学共振现象.数值结果表明：不同于一般的介质样品,金属样品的近场图像与入射光频率直接相关,改变入射光的频率,获得的样品近场图像的形状和对比度都会发生变化.特别是当入射光频率处于样品极化共振范围内时,金属纳米粒子的极化率会出现光极化共振,这样就可以获得样品粒子的最大有效尺寸,为提高系统的分辨率提供了一条重要途径.     

Full range spectral domain optical coherence tomography using a fiber-optic probe as a self-phase shifter

We present a full range handheld probe type spectral domain optical coherence tomography (SD-OCT) method. Here, the sample arm is composed of a tilted fiber-optic cantilever scanner; thus, the phase shift concurrently occurs while sample scanning. With the phase shift, we could achieve a full range complex-conjugate-free OCT image with no additional phase shifters in the reference arm. To realize this technique, a magnetically actuated probe was adopted. Full range SD-OCT images of a pearl, human fingernail, and human tooth were subsequently obtained using this suggested probe. The scanning range and acquisition speed were 3?mm and 20 frames/s?, respectively.     

Assessing the structure and function of single biomolecules with scanning transmission electron and atomic force microscopes

The scanning transmission electron microscope (STEM) and the atomic force microscope (AFM) have provided a wealth of useful information on a wide variety of biological structures. These instruments have in common that they raster-scan a probe over a sample and are able to address single molecules. In the STEM the probe is a focused electron beam that is deflected by the scan-coils. Detectors collecting the scattered electrons provide quantitative information for each sub-nanometer sized sample volume irradiated. These electron scattering data can be reconstituted to images of single macromolecules or can be integrated to provide the mass of the macromolecules. Samples need to be dehydrated for such quantitative STEM imaging. In contrast, the AFM raster-scans a sharp tip over a sample surface submerged in a buffer solution to acquire information on the sample's surface topography at sub-nanometer resolution. Direct observation of function-related structural changes induced by variation of temperature, pH, ionic strength, and applied force provides insight into the structure-function relationship of macromolecules. Further, the AFM allows single molecules to be addressed and quantitatively unfolded using the tip as nano-tweezers. The performance of these two scanning probe approaches is illustrated by several examples including the chaperonin GroEL, bacterial surface layers, protein crystals, and bacterial appendices.     

Image dipole approach and polarization effects in scanning near-field optical microscopy

A coupled-dipole approach is proposed in order to study the coupling between the probe tip and the rough sample in SNOM. In the present model both the optical probe tip and the sample protrusions are represented by polarizable dipole spheres. The induced polarization effects on the sample surface can be replaced by the image dipoles in the circumstance of quasi-static electromagnetic field approximation. Applying the radiation theory of the dipole, we have established a set of self-consistent equations to describe the field distribution at the sites of the probe tip and the sample protrusions. The results are completely the same as those obtained by means of the dyadic electromagnetic propagator formalism and also the derivation procedure is relatively simple. This method permits us to analyze the physical mechanisms of the interaction between the probe tip and the rough surface in SNOM intuitively. Based on this approach, we further discuss the influence of polarization of the incident light on the imaging quality. The calculating result shows that the shape and the contrast of the images of the sample are both sensitive to the field polarization, and the z-polarized mode is proved to give better resolution in SNOM.     

光子扫描隧道显微镜的光电成像系统研究

本文论述了光子扫描隧道显微镜（ＰＳＴＭ）的显微成像机理、成像规律，针对具体的物理模型进行数值模拟计算，并得到了与实际探测相一致的场分布规律。采用自行研制的光子扫描隧道显微镜（ＰＳＴＭ）的显微实验系统对多种样品进行了表面显微成像研究，获得了关于样品表面三维立体图像信息，通过多种图像处理手段对原始图像进行后期处理，得到了更具视觉效果、更为逼真的样品表面图像。     

Tribological behavior of micro/nano-patterned surfaces in contact with AFM colloidal probe

In effort to investigate the influence of the micro/nano-patterning or surface texturing on the nanotribological properties of patterned surfaces, the patterned polydimethylsiloxane (PDMS) surfaces with pillars were fabricated by replica molding technique. The surface morphologies of patterned PDMS surfaces with varying pillar sizes and spacing between pillars were characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). The AFM/FFM was used to acquire the friction force images of micro/nano-patterned surfaces using a colloidal probe. A difference in friction force produced a contrast on the friction force images when the colloidal probe slid over different regions of the patterned polymer surfaces. The average friction force of patterned surface was related to the spacing between the pillars and their size. It decreased with the decreasing of spacing between the pillars and the increasing of pillar size. A reduction in friction force was attributed to the reduced area of contact between patterned surface and colloidal probe. Additionally, the average friction force increased with increasing applied load and sliding velocity.     

检测Fe~(3+)/F~-的双功能喹哪啶衍生物荧光探针

对一种简单结构的喹哪啶衍生物作为离子荧光探针的性能进行了研究。探针由8-羟基喹哪啶的2-位引入水杨醛构成,通过双键连接喹啉环与苯环以及推-拉电子基团构成大共轭结构,使其发光量子产率提高;探针分子中的氮、氧原子提供了良好的配位作用点,能选择性与离子配合而使荧光性质发生变化。在乙腈/水溶液中,Fe~(3+)与探针形成1∶1的配合物而使其荧光猝灭,配合为自发的熵驱动放热过程。红外光谱和1 H NMR滴定推测探针分子中的两个羰基氧和氮的孤对电子参与Fe~(3+)络合,光诱导引发电子转移过程导致荧光猝灭。在乙腈溶液中,F~-使探针在415nm处的荧光峰降低,在560nm处出现新荧光峰,形成比率荧光,荧光由蓝色变为黄色至橙红色。同时,F~-使探针在280和340nm处的紫外吸收峰降低,在455nm处出现新的吸收峰,形成比率吸收,颜色由无色变为黄色至橙色。1 H NMR滴定推测探针分子与F~-是通过氢键作用。为一种同时检测阴、阳离子的双功能探针,荧光法对Fe~(3+)和F~-的检出限分别低至13.6nmol·L~(-1)和1.6μmol·L~(-1),紫外法对F~-的检出限低至16.5μmol·L~(-1)。利用探针对F~-识别时明显的颜色变化,建立了可视性,快速度,易操作的目视检测微量F~-的方法。     

JPEG图像压缩对测量类图像的影响

在JPEG图像压缩的量化过程中,丢弃了部分高频信息,使图像的清晰度下降。通过对频谱的对照分析和对实际图像进行实验后表明,JPEG图像压缩对测量和处理结果的影响与原图像的特点、取样方式、滤波过程和压缩算法有关。经摄像机和图像卡获得的图像,其压缩比可达到10;经扫描仪获得的图像,其压缩比可达到6;经数码相机获得的图像,其压缩比可达到6～8;对特定的医学图像、晶相图像等来说,利用图像的特征构造最优匹配的JPEG压缩量化表,其压缩比可达到10。采用合适的压缩比和改进的压缩方法,图像的细节损失可以控制在较小的范围内,基本上不影响检测类图像的边界提取、分割、尺寸测量时的处理结果。     

Micro-computed tomography observation of sublimation interface and image analysis on sublimation process during freeze-drying

The freeze-drying process is complicated with complex heat and mass transfer during sublimation. The sublimation interface of freeze-drying has become more attractive and meaningful recently. In this study, apple slices undergoing sublimation were scanned by a Micro-CT scanner. The cross-sectional images were reconstructed with those scanning images respectively. The technique of grey value analysis was used to recognize the procedure. The results showed that, from direct scanning images and 2-D reconstructed images, a 3-D moving mode of sublimation interface which contracted to the centre of the sample could be seen, sublimation process proceeded from edge to center gradually. The grey value of ice crystals was determined to be 154 through gauss calculation. By comparing frozen sample with freeze-dried one, the ice crystals regions in the beginning became the porous regions after drying, grey values increased correspondingly. Samples shrunk slightly after drying for 3 to 7 hours, which could be distinguished by the change in grey values.     

Nonlinear optical probe of tunable surface electrons on a topological insulator

We use ultrafast laser pulses to experimentally demonstrate that the second-order optical response of bulk single crystals of the topological insulator Bi(2)Se(3) is sensitive to its surface electrons. By performing surface doping dependence measurements as a function of photon polarization and sample orientation we show that second harmonic generation can simultaneously probe both the surface crystalline structure and the surface charge of Bi(2)Se(3). Furthermore, we find that second harmonic generation using circularly polarized photons reveals the time-reversal symmetry properties of the system and is surprisingly robust against surface charging, which makes it a promising tool for spectroscopic studies of topological surfaces and buried interfaces.     

双探头三维表面轮廓测量系统

提出了一种将原子力显微(AFM)探头与位置敏感探测器测距探头相结合的双探头三维表面轮廓测量新方法, 可在获取样品表面轮廓的同时, 测定样品局部形貌。搭建了双探头三维表面轮廓测量系统, 阐述了系统的工作原理, 并对其结构组成包括双探头、步进扫描台和计算机控制平台进行了说明。用2000 line/mm的光栅进行了扫描实验, 对系统的测量范围进行了标定。以外径8 mm、内径4 mm的金属垫圈为样品, 进行了整体三维表面轮廓与局部表面形貌测量实验, 给出了垫圈表面图和局部三维形貌图。结果表明, 该系统能满足不同尺寸和材质的样品的测量要求, 即可实现对样品轮廓的大范围扫描测量, 又可对样品局部进行高精度形貌测量。     

Analysis of roughness of Cs surfaces via evaluation of the autocorrelation function

At low temperature prepared quench-condensed Cs surfaces are analysed on a nanometer scale via scanning tunneling microscopy. The analysis of surface roughness is presented with the help of the evaluation of their autocorrelation function. In order to extract the correct autocorrelation function we present the requirement regarding the scan resolution of scanning probe microscopy (SPM) images in general. This is supported by a ‘numerical experiment’. Furthermore, we present some methods of deducing higher orders of autocorrelation lengths, which are needed to evaluate SPM images with non-random distribution of roughness amplitudes. These characteristic values of the autocorrelation function could play the key role in further statistical calculations, e.g., on how surface roughness alters the wetting behaviour of liquid helium adsorbed on the cesium surfaces.     

Real-time imaging of the spatial distribution of rf-heating in NMR samples during broadband decoupling

Continuous radio-frequency (rf) irradiation during decoupling and spin-lock periods in NMR pulse sequences may lead to undesired sample heating. Heat-sensitive samples can suffer damage from the sudden temperature rise which cannot be adequately compensated by the temperature control system. Moreover, as the heating is spatially inhomogeneous, higher temperature increases can arise locally than are indicated by the average increase detected by the temperature controller. In this work we present a technique that allows measurement of a real-time 2D-image of the temperature distribution inside an NMR sample during an experiment involving rf-heating. NMR imaging methods have previously been used to project the temperature distribution inside an NMR sample onto a single spatial axis or to acquire steady-state 2D- temperature distributions. The real-time 2D-temperature profiles obtained with our procedure provide much more detailed data. Our results show, that not only inhomogeneous heating but also inhomogeneous sample cooling contribute to the build-up of temperature gradients across the sample. The technique can be used to visualize rf-heating in order to protect sensitive samples and to experimentally test new coil geometries or to guide probehead design.     

Spin-polarized nuclei as probes of electromagnetic field distributions on solid surfaces

Recent experiments using thermal beams of nuclear-spin-polarized alkali atoms adsorbed on hot metal surfaces show that polarized nuclei are sensitive probes of surface electromagnetic field distributions. The high polarization of the probe beams, when coupled with the efficiency of atomic physics techniques used for monitoring the polarization of desorbed particles, makes possible a variety of interesting spinrelaxation experiments on single-crystal surfaces, including nuclear magnetic resonance. Extension of the current experimental method to semiconductor and insulator surfaces at arbitrary temperatures appears to be straightforward. The information from spin-polarized nuclear surface spectroscopy (SPNSS) will allow detailed tests of charge-density profiles now available in self-consistent surface structure calculations. Moreover, the variety of presently available polarized nuclear species suggests that the chemistry of many interesting adsorbate-surface systems could also be profitably investigated by this technique. The use of spin-polarized hydrogen nuclei in particular offers enticing prospects for fundamental studies in catalysis, surface structure and basic two-dimensional physics.     

扫描探针声显微镜轻敲模式中相位像的反差机理

根据扫描探针声显微镜(SPAM)轻敲工作模式中探针作周期振动的特点,以及在探针与试样相接触过程中激振力和悬臂探针自由振动的阻尼力很小的假设下,解析求解了探针与试样的碰撞运动方程,得到了最大压痕深度和碰撞时间与探针半径、等效杨氏模量以及界面吸附能等之间的关系式,较直观地说明了SPAM中轻敲模式的相位像反差机理:信号的相位与试样微区的性质、探针振幅、设置点以及试样形貌等有关。并定量预计了纳米金刚石像中的相位差值72.59°,与实验测量平均值73.2°±8.2°一致。同时,合理地解释了实验得到的光学薄膜试样相位像的反差。这些表明SPAM轻敲模式的相位成像是一种纳米分辨率测量材料表面物理性质的成像技术。     

Magnetic resonance force microscopy combined with surface topography

A new method of surface microscopy is proposed, which combines three-dimensional electron spin resonance imaging by magnetic resonance force microscopy (MRFM) and topographic imaging of the sample surface by scanning force microscopy (SFM). In order to demonstrate its potential for the identification of microscale objects, the individual and combined images are used to provide the locations, shapes and spin density distributions of target phantom objects. We report spatial resolution in MRFM of 2.8 x 2.8 x 2.0 microm(3). This could be improved to the theoretical limit of 0.08 x 0.08 x 0.04 microm(3) through reduction of the thermal noise by cooling to cryogenic temperatures approximately 0.5K. We believe that this type of microscopy will become a very useful tool for the investigation of anomalies induced in surfaces by materials buried below the surface.     

Coloration of a metal surface under pulsed laser irradiation

The regimes of irradiation using nanosecond laser pulses for creation of color images on stainless-steel and titanium surfaces upon laser engraving are studied. Parameters of radiation that correspond to the spectrum of resulting colors on the sample surface are experimentally determined. The spectral analysis of the irradiated area is performed and probe microscopy is used to study the surface relief. Complicated surface relief that results from irradiation indicates the contribution of several optical effects responsible for the surface color under laser engraving.     

Hyperfine-interaction studies of surfaces

Applications of hyperfine-interaction techniques, like NMR, PAC and Mößbauer spectroscopy, to well-characterized surfaces are discussed and the present knowledge of surface hyperfine fields is reviewed. Measurements of nuclear spin relaxation permit to extract the local density of electron states at the Fermi level of adsorbed alkali atoms. From the observed electric-field-gradient properties surface probe sites and diffusion processes can be inferred; the experimentally determined magnetic hyperfine fields give access to the electron-spin behaviour at magnetic surfaces.