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1、九、扫描探针显微镜九、扫描探针显微镜（Scanning probe microscopy）近代分析实验原理（Introduction of modern analytical methods）12BiFeO3薄膜的SEM照片BiFeO3纳米线的AFM照片3scanning tunneling microscope (STM)scanning force microscope (SFM)atomic force microscope (AFM)4STM，1982Nobel Prize, 1986IBMAtomic force microscope (AFM), 1985With Quate in
2、 Stanford university561. InstrumentationThe main components of a scanning probe microscope (SPM).78The scanning tunneling microscope (STM) system. The scanner moves the tip over the sample surface and the feedback loop keeps the tunneling current constant.1.1 Probe and ScannerThe probe is the basic
3、SPM component that directly interacts with a sample surface to be examined or measured.tungsten wire for STMSiO2 or Si3N4 and mounted onto a cantilever spring for atomic force microscope (AFM)9The scanner controls the probe that moves over the sample in three dimensions in a precise way. The scannin
4、g probe needs to be positioned with an accuracy of 1 pm (1012 m) if atomic resolution is required. To achieve this level of precision, a scanner is made from piezoelectric materials.Piezoelectric materials change shape in an electric field or generate an electric field due to shape change. The relat
5、ive length of the change is proportional to applied voltage (E).piezoelectric coefficienttypical value is about 0.3 nm V1Greater than 300 volts is required to achieve 1 m movement for a 10 mm long piezoelectric tube.101.2 Control and Vibration IsolationBy computer with feedback control systemCollect
6、 dataFor high resolution, its performance relies on precise control of the distance between the mechanical probe and sample surface.vibrationfloor vibrationsacoustic vibrationby mounting the SPM on springs, rubber feet or air-damped feetby operating the instrument in a solid box112. Scanning Tunneli
7、ng Microscopy (STM)2.1 Tunneling Currentbias voltage applied between the tip and the surfacethe nearest distance between the tip and the sample surface(typically between 1mV and 4V)when the gap is on the scale of interatomic distance(10 pA - 10 nA)proportional to the local density of states (LDOS) o
8、f the sample at the Fermi level.a surface profile generated by an STM may differ from the surface topography of sample.12GN1N2+ N1N2G N1N2+ N1N2定义电子的自旋极化率：P=(N-N)/(N+N)TMR=(R-R)/R=(G-G)/G=2P1P2/(1-P1P2)132.2 Probe Tipstungsten or PtIr alloyselectrochemical etching methodmechanical cutting or grindin
9、gThe surface oxide formedon the tip during etching must be removed.142.3 Working Environmentsin air and liquid environments (can)ultra-high vacuum (most), low temperature (preferred)most sample surfaces in an air environment quickly develop an oxide layer. An ultra-high vacuum environment can preven
10、t possible surface oxide formation and maintain a conductive sample surfacereduces thermal drift and diffusion of atoms and helps to obtain a static surface image of atoms. However, an elevated temperature provides an environment for observing dynamic processes of atoms and molecules.152.4 Operation
11、al Modesconstant current, constant height, spectroscopic and manipulation modes2.4.1 constant current modemost commonly used162.4.2 constant height modemuch higher scanning rates172.4.3 spectroscopic moderecording the tunneling current as a function of either tipsample spacing or bias voltage.studyi
12、ng surface properties such as superconduction and molecular adsorbtion on metal.the tip is stopped at a position above the sample, and the tunneling current is recorded while changing either the spacing or bias voltage between the tip and sample.2.4.4 manipulation modeManipulation mode in the STM: (
13、a) vertical manipulation, where an adatom forms a strong bond with the tip and is detached from the surface, then it is transported by the tip and redeposited on the surface; and (b) lateral manipulation where the tip forms a weak bond with an adatom and moves it along the line of manipulation.contr
14、olled by voltage pulses181990, IBM, 35 Xe atomsGraphite surface， moving the C atoms0.13 nm192.5 Typical Applications of STM2.5.1 image atomsAn STM image of a 32 nm36 nm area on the Si (111) planes. Surface atoms have been reconstructed, showing 77 symmetry. Three atomic steps separate terraces.annea
15、ling a silicon single crystal above 1000 oC under ultra-high vacuum202.5.2 topography of a surfaceOxygen adsorbed onto a GaAs surface: (a) the sample is under negative bias voltage (2.6 V with respect to the tip); and (b) the sample is under positive bias voltage (+1.5 V with respect to the tip).int
16、erpreted carefullyThe image may be altered by the bias voltage applied to a sample because the image reflects the surface of equal LDOS.21The main limitations:both the tip and sample surface are good conductorslimits the materials that can be studiedAtomic Force Microscopy (AFM)223. Atomic Force Mic
17、roscopy3.1 Near-Field ForcesShort-Range Forcesatomic forcesoverlapping of their electron clouds, causes either attractive or repulsive forces.0.51 nN per pair of interactive atoms between a typical tip and a sample.The decay length of the forces is in the range of 0.050.2 nm, larger than that of a t
18、unneling current23Van der Waals Forcesthe interactive forces between dipoles of molecules.assume the tip is a sphere with radius, Rdipoles that arises from thermal fluctuation or electric field inductionHamaker constant and is in the order of 1019 Jthe spacing between tip and samplea tip radius of 3
19、0 nm and vacuum gap of 0.5 nm, the van der Waals forces arearound 2 nN.Electrostatic Forcesinteractive forces between the electric charges of tip and sample.Vcpd represents the contact potential difference due to the difference in work functions of the tip and the sample.a tip radius of 20 nm and a
20、distance between sample and tip (d) of 0.5nm,the electrostatic force is about 0.5 nN when the difference between Vbias and Vcpd equals 1V24Capillary Forcesforces resulting from water vapor condensation between tip and sample.surface tension of water nucleusa tip radius of 100 nm, the maximum capilla
21、ry force reaches about90 nN.This force is significantly larger than other forces being discussed. If an attractive force in the order of 108107 N is recorded, it most likely arises from the capillary.We can reduce these forces by covering the sample with hydrophobic molecules. AFM measurements in li
22、quid are not affected by capillary forces.253.2 Force SensorsDetection of cantilever deflection by a laser beam reflected from the rear side of the cantilever. The reflection is measured by four-quadrant photodiodes. Cantilever deflection is converted to the changein beam position by photodiodes. t,
23、 l and w, cantilever dimensions; h, distance from back of cantileverto probe tip.force between the tip and sampleelastic bending of the cantileverbeam deflectionposition-sensitive photodiodes26273.3 Operational Modesstatic or dynamic modesIn static modes, the cantilever statically deflects to a cert
24、ain degree and the feedback loop maintains this set value of deflection during scanning.contact typethe tip on the cantilever physically touches the surface to be examined.In the dynamic modes, the cantilever oscillates at a designated frequency, and the feedback loop tries to maintain a set value o
25、f amplitude of oscillation during scanning.intermittent contact mode （tapping mode）reduces possible damage to a sample during scanning and also can provide information on chemical and physical properties of the surface, in addition to topography.both contact and non-contact typesmost widely used28In
26、teraction forces between a tip and sample atoms in: (a) non-contact mode; and (b) contact mode. Force versus distance between the tip and sample in: (c) non-contact mode; and (d) contact mode. The solid line marks total force between tip and sample; dashed line marks short-range force; A, oscillatio
27、n amplitude of the tip; arrows mark the jump-in and spring-off contact in the static mode; and the straight broken line marks the value of cantilever spring constant (k).attractiverepulsivestiff cantileversdynamic modesstatic modessoft cantileversstiff cantileverssoft cantilevers293.3.1 Static Conta
28、ct ModesForce balance in the contact mode. The attractive long-range forces (Fattr) are balanced by the short-range repulsive force (Frep) at the contact area and the spring force of the cantilever.The resolution of the contact mode depends on a contact area at the tip apex.it is difficult to obtain
29、 atomic resolution with the contact mode.The contactdiameter：several nanometers303.3.2 Lateral Force Microscopy(friction force microscopy)Detection of distortion of the cantilever due to lateral force between tip and sample in the lateral force microscope.a variation of the static contact mode313.3.
30、3 Dynamic Operational ModesThe dynamic non-contact mode. The frequency shift of the cantilever oscillation is used to operate the feedback loop.provide true atomic resolution and image quality comparable to an STM.this mode cannot be operated in a liquid environment.The dynamic non-contact mode323.3
31、.4 Tapping ModeThe tapping mode. The oscillation of the cantilever is dampened when the tip touches the sample surface at each cycle. The image is formed by keeping the oscillation amplitude constant.prevents the tip from sticking to the surface and from damaging the sample surface during scanning.o
32、vercome the tipsample adhesion forces.the surface material is not pulled sideways by shear forces since the applied force is always vertical.important for soft samples such as polymers and biological materials.333.3.5 Phase imagingpowerful extension of the tapping modecan detect variations in proper
33、ties including composition, adhesion, friction and viscoelasticity.Phase image in the tapping mode. The phase lag of cantilever oscillation is detected. The phase lag is very sensitive to variations in material properties such as adhesion and viscoelasticity.343.3.6 Force ModulationForce modulation.
34、 The tip is kept in contact with the sample. Differences in local properties of the sample will be reflected in the amplitude change of the cantilever.a low frequency oscillationa constant forcestiffer area on the sample will undergo less elastic deformation than a soft area. Thus, a stiffer area ge
35、nerates greater resistance to the vertical oscillation and less bending of the cantilever354. Typical Applications4.1 Static ModeTopography of crystalline polyethylene. The chain folds of the crystal structure generate the rough appearance.聚乙烯36A scanning force microscope (SFM) image (5m5m) of a mix
36、ed film of fluorocarbon and hydrocarbon carboxylates. The circular domains were assigned to the hydrocarbon component and the surrounding flat film to the partially fluorinated component: (a) topographic image; and (b) lateral force image obtained simultaneously with the topography. The measured lat
37、eral force generates higher contrast due to the difference in friction.374.2 Dynamic Non-Contact ModeA non-contact mode image of a hydrogen-terminated silicon monohydride surface. Line profile of frequency shift along the white dotted line indicates the location of hydrogen atoms. The initial freque
38、ncy of the cantilever is about 3 Hz.Si (100) surfaceultra-high vacuum of 2108 Pa384.3 Tapping ModeTapping-mode image of multiple layers of AlxGa(1x)N deposited on a sapphire substrate.394.4 Force ModulationCross-section of carbon fibers in an epoxy matrix: (a) topographic image; and (b) force modula
39、tion image. The image width is 32m.405. ArtifactsArtifacts generated by tip: (a) the side of tip causes broadening of the image of the protruding object; and (b) limits in the width the of tip limits cause distortion of the hole geometry in the image.41Artifacts are most likely to be revealed if the
40、 image changes after changing such scanning methods.42AFMMFM600 oC, annealed电子衍射MFMAs-prapared FePtFePt arrays prepared by Focused Ion Beam43The surface of SrTiO3 substrate处理前，基片的Rq为0.58nm处理后为0.14nm，处理后同一台阶内的Rq只有0.07nm。台阶高度为0.38nm，与STO的晶格常数很接近，说明得到了原子级的台阶不同取向BiFeO3单晶薄膜的铁电畴44（111）（001）surface roughness 0.24 nmsurface roughness 0.1 nmLa0.67Sr0.33MnO3BiFeO345BiFe0.5Mn0.5O3 filmBiFeO3-BiMnO3 superlattice film
