A scanner for an ultrahigh-vacuum low-temperature scanning tunneling microscope

A scanner for an ultrahigh-vacuum low-temperature scanning tunneling microscope is described. It has a high resonance frequency (>30 kHz) and a small thermal-drift rate (≤1 nm/°C) at room temperature. The scanner feeds the tip to the sample at a distance of up to 3 mm and positions it in the sample plane on a 4 × 4-mm area. These characteristics of the scanner allow one to study atomic structures at temperature variations from 5 to 300 K with objects under study remaining in view of the microscope. The scanner has a horizontal attachment for a sample with a size of up to 6 × 6 × 3mm and ensures a scanning field of 4.8 × 4.8 × 0.6 μm at 300 K and 0.8 × 0.8 × 0.1 μm at 5 K, as well as the possibility of heating to 150°C and easily replacing the sample and tip with vacuum manipulators.     

Optical Fiber Catheter with Distal End Bending Mechanism Control for Raman Biospectroscopy

Abstract

This work proposes the development of an optical catheter with bending control of the distal end. The probe consists of seven optical fibers wrapped in a resin and a biocompatible flexible teflon tube with a novel mechanical device that allows bending of the distal extremity to access a desired location of a human organ. A central fiber is used for tissue Raman excitation, five fibers are used for Raman signal collection, and the seventh for “optoclinical” treatment applications. Infrared, dispersive Raman spectra at 785 nm excitation were employed to optically characterize the proposed catheter. An excitation transmission loss of 16% was found compared to the traditional six collecting fibers catheter, both with their distal tip straight. By bending of the distal tip at different angles, with turning the intermediated section of the catheter around cylinders of different diameters (one finds a correlation between curvature angle of the tip and cylinder diameter), the transmission loss coefficient and transmission were determined for each distal tip angle. A transmission reduction of 5% was found for a 180° curvature. This optical catheter could be very useful in clinics, providing a way to control the fiber tip position and angle onto the tissue or organ.     

Apertureless near-field Raman spectroscopy

We report on the tip‐enhanced Raman spectra of C60 obtained on a custom‐built apertureless scanning near‐field optical microscope. A commercial atomic force microscope tip coated with 100 nm thickness of gold was used to enhance locally the Raman signal and permit topographic and spectral information to be acquired simultaneously. We present preliminary data which demonstrate the tip enhancement effect using C60 as a test sample.     

Plasmon-Enhanced Near-Field Optical Spectroscopy of Multicomponent Semiconductor Nanostructures

Multicomponent semiconductor nanostructures were studied by local spectral analysis based on surface-enhanced Raman scattering by semiconductor nanostructures located on the surface of an array of Au nanoclusters near the metallized tip of an atomic force microscope. In the gap between the metal nanoclusters and the tip, where a semiconductor nanostructure is located, there is a strong increase in the local electric field (hot spot), resulting in a dramatic enhancement of the Raman scattering signal. An unprecedented enhancement of the Raman scattering signal by two-dimensional (over 108 for MoS2) and zero-dimensional (106 for CdSe nanocrystals) semiconductor nanostructures was achieved. The use of the method for mapping the Raman scattering response of a multicomponent system of MoS2 and CdSe made it possible to identify components with a spatial resolution far exceeding the diffraction limit.     

Design and characterization of a PZT driven micromachining tool based on single-point tool tip geometry

This paper presents the design and characterization of a piezoelectric tube scanner (PZT) driven micromachining tool that is based on single-point tool tip geometries. The cutting module is made of a diamond stylus, which is known as the hardest material available. Therefore, this tool can be used to machine a wide range of materials ranging from polymers, through metals to glass. The microscopic cutting motion is achieved by using a piezo tube scanner as an actuator. The tool tip is attached to the free end of the PZT while the other end is firmly held stationary. By applying sinusoidal waveforms with different phase angles to the x and y electrodes of the PZT, the micro orbital motion of the diamond tip can be generated for machining purposes. This paper details the design of the tool and reports on its dynamic behavior in both short- and long-term experiments.     

Control of tip-to-sample distance in atomic force microscopy: a dual-actuator tip-motion control scheme

The control of tip-to-sample distance in atomic force microscopy (AFM) is achieved through controlling the vertical tip position of the AFM cantilever. In the vertical tip-position control, the required z motion is commanded by laser reading of the vertical tip position in real time and might contain high frequency components depending on the lateral scanning rate and topographical variations of the sample. This paper presents a dual-actuator tip-motion control scheme that enables the AFM tip to track abrupt topographical variations. In the dual-actuator scheme, an additional magnetic mode actuator is employed to achieve high bandwidth tip-motion control while the regular z scanner provides the necessary motion range. This added actuator serves to make the entire cantilever bandwidth available for tip positioning, and thus controls the tip-to-sample distance. A fast programmable electronics board was employed to realize the proposed dual-actuator control scheme, in which model cancellation algorithms were implemented to enlarge the bandwidth of the magnetic actuation and to compensate the lightly damped dynamics of the cantilever. Experiments were conducted to illustrate the capabilities of the proposed dual-actuator tip-motion control in terms of response speed and travel range. It was shown that while the bandwidth of the regular z scanner was merely a small fraction of the cantilever's bandwidth, the dual-actuator control scheme led to a tip-motion control system, the bandwidth of which was comparable to that of the cantilever, where the dynamics overdamped, and the motion range comparable to that of the z scanner.     

Resonant excitation of tip plasmons for tip-enhanced Raman SNOM

The conditions necessary for the optimisation of tip-enhanced scanning near-field optical microscopy have been determined. The Raman scattering efficiency can be enormously increased by enhancements in the local field amplitude, such as that which can occur in the vicinity of a metallic nanostructure. The field enhancement in the vicinity of a silver tip is investigated theoretically here using the finite difference time domain method. Field enhancements from electron oscillations on the tip are shown to display strong maxima at resonant illumination wavelengths and the nature of these enhancements at the substrate surface beneath the tip, both on and off resonance, has been calculated. The enhancement of the Raman signal on the surface decreases exponentially as the tip–substrate separation is increased and a peak Raman enhancement of 10⁷ is theoretically achievable at a tip–surface separation of 2 nm. The resolution is also strongly related to the distance between the tip and the substrate surface narrowing to <7 nm, significantly smaller than the radius of curvature of the end of the tip.     

Prediction of the resonant frequency of piezoelectric tube scanners through three-dimensional finite element modeling of a tube assembly

In this paper, we study the resonant behavior of a piezoelectric tube scanner used in scanning probe microscopes. In particular, we use a finite element model to allow the inclusion of boundary effects, i.e., nonrigid bonding layers and elastic tip holder, on the dynamic response of the scanner. We show that although existing analytical models provide a good estimation of the axial resonant frequency, their predictions of the first bending resonance may have up to a 100% error. In addition, a simple procedure is proposed that combines some precalculated factors to predict the lowest resonant frequency for a wide range of typical scanner dimensions. An experiment is carried out to illustrate the procedure and validate the prediction.     

Large-area topography analysis and near-field Raman spectroscopy using bent fibre probes

We present a method for combined far‐field Raman imaging, topography analysis and near‐field spectroscopy. Surface‐enhanced Raman spectra of Rhodamine 6G (R6G) deposited on silver nanoparticles were recorded using a bent fibre aperture‐type near‐field scanning optical microscope (NSOM) operated in illumination mode. Special measures were taken to enable optical normal‐force detection for control of the tip–sample distance. Comparisons between far‐field Raman images of R6G‐covered Ag particle aggregates with topographic images recorded using atomic force microscopy (AFM) indicate saturation effects due to resonance excitation.     

The role of tip plasmons in near-field Raman microscopy

The enhancement in electric field strength in the vicinity of a metal tip, through the excitation of plasma modes in the tip, is investigated using the finite difference time domain method; such tip enhancement has significant potential for application in scanning near-field Raman microscopy. To represent an experimentally realistic geometry the near-field probe is described by a conical metal tip with a spherical apex, with radii 20 nm and 200 nm considered, in close proximity to a glass substrate. Illumination through the substrate is considered, both at normal incidence and close to the critical angle, with the polarization in the plane of incidence. By modelling the frequency dependent dielectric response of the metal tip we are able to highlight the dependence on the scattering geometry of the nature of the electromagnetic excitations in the tip. In particular, the strongest electric field enhancement with the greatest confinement occurs for the excitation of modes localized at the tip apex, excited only for off-normal incidence. Bulk modes excited in the tip also produce enhancement, although over a larger area and with significantly less enhancement than that of the localized modes; however, the excitation of bulk modes is independent of the angle of incidence.     

Real-time atomic force microscopy in lubrication condition

We have studied frictional force and wear problem in real-time atomic force microscopy in contact-mode using a resonator type mechanical scanner allegedly reported. The fast scanning may cause wear in the sample surface or the tip, and may deteriorate the image quality. Mineral oil was used to make a lubricious surface on a polycarbonate sample, and it was found that the interfacial frictional force was decreased. A Si tip which was coated with a hydrophobic film by means of chemical modification was confirmed to diminish the frictional force in the fast scanning process. The resultant image quality was improved due to reduced friction and wear.     

Near-field Raman spectroscopy using a sharp metal tip

Near‐field Raman spectroscopy with a spatial resolution of 20 nm is demonstrated by raster scanning a sharp metal tip over the sample surface. The method is used to image vibrational modes of single‐walled carbon nanotubes. By combining optical and topographical signals rendered by the single‐walled carbon nanotubes, we can separate near‐field and far‐field contributions and quantify the observed Raman enhancement factors.     

原子力显微镜管式扫描器运动学建模与误差分析

针对样本扫描模式原子力显微镜,对其管式扫描器-样本-探针系统进行了运动学分析,建立了该系统的运动学模型,该模型表明:对于给定原子力显微镜扫描器,样本上与探针接触点的横向和纵向位移取决于探针尖端相对于扫描管轴心的偏置量、所加电压(或名义扫描范围)及样本厚度。据此模型,对由于弯曲运动模式所产生的两种重要误差—交叉耦合误差及扫描范围误差进行了定量分析,分析表明:扫描范围误差主要受样本厚度及名义扫描范围影响,而Z向交叉耦合误差主要受探针偏置量及名义扫描范围影响,实验验证了所建立的运动学模型和误差计算公式的正确性;另外,还提出了相应的减小误差的方法。     

Microfabrication of a combined AFM-SNOM sensor

The objective of this work is to fabricate a scanning probe sensor that combines the well-established method for atomic force microscopy, employing a micro-machined Si cantilever and integrated tip, with a probe for the optical near field. A photosensitive pn-junction is integrated into the tip for that purpose and an Al coating is applied to the tip. It comprises an aperture of 50-70 nm in diameter at the apex of the tip in order to spatially limit the interaction of the tip to the optical near field of the sample. Characterization of the tip and first results of simultaneously recorded force and photon images are presented.     

Imaging of granular high-Tc thin films using a scanning tunnelling microscope with large scan range

STM images are presented in the micron scale, taken in UHV with a single-tube scanner STM with SEM control of the tip position. For calibration, a carbon grid was used from which the coupling of x and y scan axes has been determined as well as the piezo-sensitivity factors. Images taken of a YBa₂Cu₃O_7-δ, film display the granular structure of the crystallites formed during the post-evaporation annealing. A direct comparison of STM scans with SEM micrographs was made which demonstrates the complementary information obtained by the two methods. The obtained resolution is compared to the tip shape which becomes crucial for the imaging of corrugated surfaces on the micron scale. With a simple geometric model, an attempt has been made to reconstruct the surface topography from STM scans based on the knowledge of the tip shape.     

Imaging and strain analysis of nano-scale SiGe structures by tip-enhanced Raman spectroscopy

The spatial resolution and high sensitivity of tip-enhanced Raman spectroscopy allows the characterization of surface features on a nano-scale. This technique is used to visualize silicon-based structures, which are similar in width to the transistor channels in present leading-edge CMOS devices. The reduction of the intensive far-field background signal is crucial for detecting the weak near-field contributions and requires beside a careful alignment of laser polarization and tip axis also the consideration of the crystalline sample orientation. Despite the chemical identity of the investigated sample surface, the structures can be visualized by the shift of the Raman peak positions due to the patterning induced change of the stress distribution within lines and substrate layer. From the measured peak positions the intrinsic stress within the lines is calculated and compared with results obtained by finite element modeling. The results demonstrate the capability of the tip-enhanced Raman technique for strain analysis on a sub-50 nm scale.     

压电微音叉扫描探针显微镜测头研究

压电微音叉具有良好的谐振特性，并易于实现其振动的检测。利用这些特性，与钨探针结合，构成了一种新型的表面轮廓扫描测头。该新型测头与X-Y压电工作台结合，采用与TM-AFM相同的工作原理，构成了扫描探针显微镜。介绍了压电微音叉扫描测头的构成、工作原理及主要特性，给出了所构成的扫描探针显微镜测量系统。通过实验及其结果，证明了新型测头具有高垂直分辨率、低破坏力等优点。除此之外，由于采用了有效长度大的钨探针，使大台阶微观表面的测量成为可能。     

参数相关随机信息约束下的地面脉冲激光扫描仪自校准

针对地面脉冲激光扫描仪自校准参数中存在的高相关性问题,基于全站仪误差模型提出了一种脉冲激光扫描仪自校准统一方法。该方法初始假设激光扫描仪的校准参数与全站仪的系统误差相似,通过引入估计参数的相关随机信息以及选择合适的网络配置,实现了降低参数相关性和提高参数估计可靠性的目的。通过平差方法对估计参数赋予权重信息,把所有参数都视为最小二乘平差中的观测值,权重越高参数相关性期望越低。最后,采用该方法对脉冲激光扫描仪Leica scan station实施了自校准。实验结果表明,该方法能够高精度估计大多数参数且实现了参数间的低相关性,有利于随机内部约束平差的使用;重大的系统误差是激光测距仪零误差和竖盘指标差,其中为了得到零误差的可靠估计,须以约1mm的精度确定扫描仪的位置。     

A fully automated, ‘thimble-size’ scanning tunnelling microscope

A novel, fully automated high-stability, high-eigenfrequency scanning tunnelling microscope (STM) has been developed. Its key design feature is the application of two piezoelectric ceramic tubes, one for the x-y-z motion of the tip and one for a linear motor (‘nano-worm’) used for the coarse positioning of the tip relative to the specimen. By means of the nano-worm, the tip can be advanced in steps between 16 and 0·2 nm. The walking distance is >2 mm, with a maximum speed of 2000 steps/s. The nano-worm positioning implies that this STM is fully controlled by electronic means, and that no mechanical coupling is needed, which makes operation of the STM extremely convenient. The axial-symmetry construction is rigid, small and temperature-compensated, yielding reduced sensitivity to mechanical and acoustic vibrations and temperature variations. The sample is simply placed on a piece of invar which surrounds the scanner tube and the nano-worm and is held by gravity alone. This allows for easy sample mounting. The performance of the microscope has been tested in air by imaging a variety of surfaces, including graphite and biological samples.