现代分析测试技术

现代分析测试技术 Atomic Force Microscopy(AFM) Theory: The AFM functions by scanning a sharp tip over a surface much the same way as a gramophone's needle scans a music record. The tip is held at the end of a cantilever beam shaped like a diving board. As the tip is repelled by or attracted to the surface by the intermolecular interactions between the atoms of the tip and the surface, the cantilever beam is relatively deflected. The deflection magnitude is captured by a detection system, commonly a laser that reflects onto a photo detector at an oblique surface, a feedback system raises and lowers the sample to keep a constant force between the tip and the surface. A plot of this upward and downward motion as function of the tip position on the sample surface provides a high-resolution image of the surface topography. This mode of operation is called “contact mode”. Alternatively, the tip can vibrate rapidly up and down and tap the surface while scanning it. This mode, referred to as “tapping”,“semicontact”, or “dynamic” mode, is actually the most common mode of operation. In the AFM, the picture on the screen represents the surface topography, and not light transmission and [1]reflection as in an optical microscope. Figure 1. Schematic diagram showing the operating principles of the AFM in the contact mode. Characteristics: AFM provides a number of advantages over conventional microscopy techniques. AFMs probe the sample and make measurements in three dimensions, x, y, and z (normal to the sample surface), thus enabling the presentation of three-dimensional images of a sample surface. This provides a great advantage over any microscope available previously. With good samples (clean, with no excessively large surface features), resolution in the x-y plane ranges from 0.1 to 1.0 nm and in the z direction is 0.01 nm (atomic resolution). AFMs require neither a vacuum environment nor any special sample preparation, and they can be [2]used in either an ambient or liquid environment. Compare with scanning tunneling microscope (STM) , AFM could achieve extremely high resolutions also on insulating samples. Moreover, the AFM allows one to manipulate matter and measure forces, constituting an “all-in-one” [3] workbench for studying matter at the nanoscale.With these advantages AFM has significantly impacted the fields of materials science, chemistry, biology, physics, and the specialized field of semiconductors. Appliations: [4]Carlos S. Ruiz-Vargas et al use atomic force microscopy to image grain boundaries and ripples in graphene membranes obtained by chemical vapor deposition. First, they characterize the topography and structure of the CVD graphene membranes using AFM. Figure 2. (a) AFM height and (b) phase tapping modeimages of a suspended graphene membrane, with grain boundaries clearly visible in the phase image. A region in the image in (a) is shown enlarged in the z direction (height) to accentuate the rippling in graphene. Scale bar:500nm. Figure 2a shows the topography of one suspended membrane, which is clamped on all sides. The membrane appears taut at its edges, without the presence of slack or major corrugations. Detailed imaging, however,reveals that the membranes are rippled on the nanometer scale (Figure 2a, inset). The surface roughness of these sheets is ?3 nm (rms), with ripples measuring a few nanometers in amplitude. It is likely that the observed ripples in our membranes are the result of a combination of factors inherent in the growth and transfer process. CVD graphene is not flat to begin with, as it is grown on a copper substrate with surface roughness comparable to that of the graphene membranes . This inherent nonflat topography could then in turn lead to rippling once the membrane is transferred and subjected to the constraints imposed by the edges of the holes in the supporting substrate. They also performed a series of nanoindentation measurements to measure the mechanical properties of graphene membranes. [5]Shaida Ibrahim and Takashi Ito studyed the surface chemical properties of ca.20 nm wide domains on a UV-treated thin film of a Polystyrene-poly(methyl methacrylate) diblock copolymer (PSb-PMMA). They observed ferritin adsorption with tapping-mode atomic force microscopy (TM-AFM). 2Figure 3. Typical TM-AFM topography image (500 ×500nm, total gray scale range: 50 nm; scan rate: 2Hz) of a UV/AcOH-treated PS-b-PMMA film upon deposition of ferritin molecules. A ferr- itin solution (1 mg/mL) in 0 1 mM phosphate buffer of pH 6.5 was cast on the film for 30 s for the deposition of ferritin.. After rinsing with water, the sample was dried and imaged using TM- AFM in air. Ferritin molecules were observed as bright dots of 12-17 nm in diameter and 6-10 nm in height, in addition to their larger aggregates. Interestingly, the majority of ferritin (81? 5%, determined from three separate samples) adsorbed onto the ridges (i.e., PS domains). The preferential adsorption of ferritin onto the PS domains could be explained on the basis of the larger electrostatic repulsion by the deprotonated acidic groups within the etched PMMA domains and also the more hydrophobic nature of the PS domains. Reference: [1] Ron Blonder; Ernesto Joselevich; Sidney R. Cohen. Atomic Force Microscopy: Opening the Teaching Laboratory to the Nanoworld. Journal of Chemical Education.2010,12,1290. [2] Cherylr.blanchard.Atomic force microscopy.The Chemical Educator.1996,1,2~3. [3] G. Binnig; C. F. Quate; Ch. Gerbe. Atomic Force Microscope. Phsycal Review Letter.1986,56,930. [4] Carlos S. Ruiz-Vargas, Houlong L. Zhuang, Pinshane Y. Huang,etal. Softened Elastic Response and Unzipping in Chemical Vapor Deposition 2263. Graphene Membranes. Nano Lett. 2011, 11, 2259– [5] Shaida Ibrahim ; Takashi Ito. Surface Chemical Properties of Nanoscale Domains on UV-Treated Polystyrene-Poly(methyl methacrylate) Diblock Copolymer Films Studied Using Scanning Force Microscopy. Langmuir 2010, 26(3), 2119–2123.