SPM: Scanning Probe Microscopy

Transcript

1 SPM: Scanning Probe Microscopy

2 Background First scanning probe microscope invented in 1981 by Binning and Roher Wide range of applications Topography/Atomic Structure Magnetic/Electric fields Surface temperatures

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4 Electron tunneling Typical quantum phenomenon Tunneling definition Wave-particle impinging on barrier Probability of finding the particle beyond the barrier The particle have tunneled through it Role of tunneling in physics and knowledge development Field emission from metals in high E field ( Fowler-Nordheim 1928) Interband tunneling in solids (Zener 1934) Field emission microscope (Müller 1937) Tunneling in degenerate p-n junctions (Esaki 1958) Perturbation theory of tunneling (Bardeen 1961) Inelastic tunneling spectroscopy (Jaklevic, Lambe 1966) Vacuum tunneling (Young 1971) Scanning Tunneling Microscopy (Binnig and Rohrer 1982)

5 Electron tunneling Elastic Energy conservation during the process Intial and final states have same energy 1D Planar Metal-Oxide-Metal junctions Inelastic Energy loss during the process Interaction with elementary excitations (phonons, plasmons) 3D Scanning Tunneling Microscopy Rectangular barriers Planar Metal-Oxide-Metal junctions 3D Scanning Tunneling Microscopy Time independent Matching solutions of TI Schroedinger eq Time-dependent TD perturbation approach: (t) + first order pert. theory

6 Time independent 2 2 d V 2m dz 0 2 E Region 1 1 e ikz Re ikz k 2 2 me 2 Plane-wave of unit amplitude traveling to the right+ plane-wave of complex amplitude R traveling to the left Region 2 2 Ae xz Be xz x 2m( V 2 0 ) 2 E exponentially decaying wave Region 3 3 Te ikz plane-wave of complex amplitude T traveling to the right. The solution in region 3 represents the transmitted wave T 2 transmissi on probabilit y

7 xs s 2m( V 0 E ) 2 T ( k 2 k 2 x 2 e 2xs x ) Barrier width s = 0.5 nm, V 0 = 4 ev T ~ 10-5 Barrier width s = 0.4 nm, V 0 = 4 ev T ~ 10-4 Extreme sensitivity to z The transmission coefficient depends exponentially on barrier width

8 Exponential dependence of tunneling current

9 Scanning Tunneling Microscopy (STM) Design and instrumentation Approach mechanism Enables the STM tip to be positioned within tunneling distance of the sample High precision scanning mechanism Enables the tip to be rastered above the surface Control electronics Control tip-surface separation Drive the scanning elements Facilitate data acquisition. Vibration isolation The microscope must be designed to be insensitive or isolated from ambient noise and vibrations. Review of Scientific Instruments 60 (1989) 165 Surface Science Reports 26 (1996) 61

10 Vibration isolation It is essential for successful operation of tunneling microscopes. This stems from the exponential dependence of the tunneling current on the tip-sample separation. Typical surface corrugation is nm or less tip - sample distance must be maintained with an accuracy of better than nm = 1 pm Dz Design criteria: The system response to external vibrations and internal driving signals is less than the desired tip sample gap accuracy throughout the bandwidth of the instrument. STM sensitivity to external and internal vibrational sources: Structural rigidity of the STM itself Properties of the vibrational isolation system Nature of the external and internal vibrational sources

11 High precision scanning mechanism Scanning Tunneling Microscopy (STM) Design and instrumentation Enables the tip to be rastered above the surface Typical piezoelectric ceramic is PZT-5H (lead zirconate titanate) Large piezoelectric response (~ 0.6 nm/v). Tube better than tripodes due to higher m in-plane tip motion the outer electrode is sectioned in 4 equal segments x and y directions given by applying differential scan signals (V x+, V x- = - V x+ ; V y+, V y- = - V y+ ) Z- motion common mode signals (V x+ = V x- ; V y+ = V y- ) applied to the electrodes allows extension of the tube in the z direction The voltages are referenced to the constant potential applied to the electrode located on the inner surface of the tube.

12 Scanning Tunneling Microscopy (STM) Design and instrumentation Bimorph cells Two plates of piezoelectric material glued together with opposite polarization vectors Applying V one plate will extend, the other will be compressed, resulting in a bend of the whole element Four sectors for electrodes Allow to move along the Z axis and in the X, Y plane using a single bimorph element

13 Scanning Tunneling Microscopy (STM) Examples of STM Apparatus STM scanner

14 Raster the tip across the surface, and using the current as a feedback signal. The tip-surface separation is controlled to be constant by keeping the tunneling current at a constant value. The voltage necessary to keep the tip at a constant separation is used to produce a computer image of the surface.

15 Constant current imaging Unchanged Tunneling Current (na) Dz Unchanged Tunneling Current (na) Typical working mode Constant height imaging Higher Tunneling Current (na) Lower Tunneling Current (na) Applied only on very flat regions

16 Spectroscopy local variation of can be studied by taking the derivative of the current as a function of the tip distance with lock-in techniques. I-V curves: Stop the feedback loop V ramp local DOS vs E local electronic structure

17 CITS (Current Imaging Tunneling Spectroscopy) Local electronic properties Apparent Topography: Simultaneous measurements of I(V,x,y) and z(x,y) During the scan: disable the feedback - ramp V and measure I(V) The ensemble of I values acquired on the surface at a chosen Vi will form a current image Each current image yields a visualization of the electronic density at a selected energy

18 Since you are measuring the electronic states, images of the same surface can vary!

19 In prima approssimazione l immagine STM è quella della densità elettronica delle superficie al livello di Fermi ad una distanza di alcuni A dalla superficie. I calcoli permettono di dimostrare che nel caso dei metalli i punti a più alta densita corrispondono alle posizioni dei nuclei mentre nel caso dei semiconduttori dipende dal legame (covalente, ionico ). Non solo, la presenza del gap di energia rende necessario applicare un potenziale di bias dal cui segno dipende il verso della corrente. Inoltre i calcoli dimostrano che sulle superfici metalliche la corrugazione e tale che per avere risoluzione atomica e necessario avvicinare molto la punta andando a basse V o alte I. L approssimazione fatta in prima approssimazione non è più valida le forze punta-campione e la configurazione esatta degli stati della punta non sono più trascurabili.

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21 Quando considero le dimensioni finite della punta è chiaro che l immagine che ottengo non è direttamente il profilo ma è la convoluzione tra la superfficie e la punta. Tipici valoridel raggio di curvatura sono 10nm.

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23 Instrumentation details STM tip: atomically sharp needle and terminates in a single atom Pure metals (W, Au) Alloys (Pt-Rh, Pt-Ir) Chemically modified conductor (W/S, Pt-Rh/S, W/C ) Preparation of tips: cut by a wire cutter and used as is cut followed by electrochemical etching Electrochemical etching of tungsten tips. A tungsten wire, typically 0.25 mm in diameter, is vertically inserted in a solution of 2M NaOH. A counter electrode, usually a piece of platinum or stainless steel, is kept at a negative potential relative to the tungsten wire. The etching takes a few minutes. When the neck of the wire near the interface becomes thin enough, the weight of the wire in electrolyte fractures the neck. The lower half of the wire drops off.

24 Fe/Cu(111)

25 Fe/Cu(111)

26 Sequenza realizzata usando atomi di Xe su una superficie di Ni(110)

27 AFM: Atomic Force Microscopy

28 La Forza Elastica (legge di Hooke): F kdx K = costante elastica della molla legata alla frequenza di vibrazione dalla relazione: k m 2 Per gli atomi in un solido = Hz ovvero kat=10n/m Un pezzo di Alluminio lungo 4mm ha una k=1n/m < kat

29 Katomico=10N/m Kcantilever<1 N/m The Atomic Force Microscope utilizes a sharp probe moving over the surface of a sample in a raster scan. The probe is a tip on the end of a cantilever which bends in response to the force between the tip and the sample.

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32 La forza che agisce sulla punta puo essere repulsiva (contact mode) oppure attrattiva (non contact mode). Nel regime di NON CONTATTO le forze sono piu deboli, variano piu lentamente, sono attrattive tipo forze di Van der Waals

33 Forces can be explained by e.g. van der Waals forces approximated by Lennard- Jones potential

34 1. Laser deflected off cantilever 2. Mirror reflects laser beam to photodetector 3. Photodetector dual element photodiode that measures differences in light intensity and converts to voltage 4. Amplifier 5. Register 6. Sample 7. Probe tip that scans sample made of Si 8. Cantilever moves as scanned over sample and deflects laser beam

35 Scanner. The movement of the tip or sample in the x, y, and z-directions is controlled by a piezo-electric tube scanner, similar to those used in STM. For typical AFM scanners, the maximum ranges for are 80 mm x 80 mm in the x-y plane and 5 mm for the z-direction. Feedback control. The forces that are exerted between the tip and the sample are measured by the amount of bending (or deflection) of the cantilever. By calculating the difference signal in the photodiode quadrants, the amount of deflection can be correlated with a height. Because the cantilever obeys Hooke's Law for small displacements, the interaction force between the tip and the sample can be determined.

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37 Feedback control is used to maintain a set force between the probe and the sample.

38 AFM: modalità altezza costante (misura ad anello aperto) Durante scansione se mantengo punto V ad un altezza costante deformazioni del supporto seguono profilo della superficie analizzata Corrente in uscita dal rivelatore di posizione del fascio dipende da forza agente sulla punta. Posso usare la corrente per ricostruire l immagine della superficie del campione. Questo metodo di acquisizione fornisce una mappa del profilo della superficie del campione (vale per piccole deformazioni).

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40 Confronto tra modalità «altezza costante» e «forza costante» Metodo «forza costante»: Permette maggiore dinamica (non c è limitazione alla deformazione massima consentita) ed una maggiore linearità (quella dello scanner non dipende da dipendenza della formula). Per contro minore velocità e minore sensibilità. Possibili due modalità regime di «contatto» e regime di «non contatto»: In non contatto devo usare punta con maggiore forza elastica (devo evitare che punta sia «risucchiata») quindi minore sensibilità Possibile soluzione è l uso di tecniche di risonanza

41 Metodo di rivelazione a forza costante (tecnica non risonante)

42 Metodo di rivelazione in regime non contatto: tecnica di risonanza Metto leva in oscillazione intorno a frequenza di risonanza propria del sistema ( 0 ) e misurare: Variazione di ampiezza A Variazione frequenza di risonanza 0 In assenza di gradiente In presenza di gradiente

43 In altre parole la cantilever viene messa in oscillazione ad una frequenza vicina a quella propria di risonanza da un elemento piezoelettrico, e avvicinata alla superfice del campione. Il segnale ottenuto dal sensore di deflessione viene analizzato con la tecnica lockin. Un circuito di retroazione agisce in modo da mantenere costante o la differenza di fase o di ampiezza fra il segnale del sensore e quello di eccitazione. Questa modalità di acquisizione mantiene costante la frequenza di risonanza e non la deflessione della cantilever, e si ottengono linee di gradiente di forza costante. La punta non tocca il campione durante la misura, in questo modo si minimizzano deformazioni e forze laterali. Poiché la distanza di lavoro dalla superfice può variare da alcuni nanometri a decine o centinaia, si possono acquisire immagini associate a forze a lungo range come quelle elettrostatiche e magnetiche assieme all immagine topografica

44 Se definisco Q come fattore di merito La variazione dell ampiezza ad una frequenza prossima ad una frequenza laterale In alternativa si può misurare agganciamento in frequenza. Tipici valori di A sono m.

45 Accessible via TappingMode Oscillate the cantilever at its resonant frequency. The amplitude is used as a feedback signal. The phase lag is dependent on several things, including composition, adhesion, friction and viscoelastic properties. Identify two-phase structure of polymer blends Identify surface contaminants that are not seen in height images Less damaging to soft samples than lateral force microscopy

46 Metodo di rivelazione in regime non contatto: tecnica di risonanza caratteristiche della misura Essendo F più deboli nel modo di misura in non contatto posso misurare campioni soffici. Ho però maggiore distanza punta-campione, ciò significa che sono necessarie punte più sottili.

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48 Contact Mode Advantages: High scan speeds The only mode that can obtain atomic resolution images Rough samples with extreme changes in topography can sometimes be scanned more easily Disadvantages: Lateral (shear) forces can distort features in the image The forces normal to the tip-sample interaction can be high in air due to capillary forces from the adsorbed fluid layer on the sample surface. The combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft samples (i.e. biological samples, polymers, silicon) due to scraping TappingMode AFM Advantages: Higher lateral resolution on most samples (1 to 5nm) Lower forces and less damage to soft samples imaged in air Lateral forces are virtually eliminated so there is no scraping Disadvantages: Slightly lower scan speed than contact mode AFM

49 The probe is scanned sideways. The degree of torsion of the cantilever is used as a relative measure of surface friction caused by the lateral force exerted on the probe. Identify transitions between different components in a polymer blend, in composites or other mixtures This mode can also be used to reveal fine structural details in the sample.

50 Compositepolymer imbedded in a matrix 1 micron scan Bond pad on an integrated circuit Contamination 1.5 micron scan MoO 3 crystallites on a MoS 2 substrate 6 micron scan

51 Special probes are used for MFM. These are magnetically sensitized by sputter coating with a ferromagnetic material. The cantilever is oscillated near its resonant frequency (around 100 khz). The tip is oscillated 10 s to 100 s of nm above the surface Gradients in the magnetic forces on the tip shift the resonant frequency of the cantilever. Monitoring this shift, or related changes in oscillation amplitude or phase, produces a magnetic force image. Many applications for data storage technology

52 Overwritten tracks on a textured hard disk, 25 micron scan Domains in a 80 micron garnet film

53 Effect of Shape of Tip

54 Details of Parts of AFM Cantilever and Tip Property Typical Value Desired Quality Material Silicon, Silicon Nitride, Silicon Oxide Hard, Unreactive Tip Radius < 10 nm Small Tip Height µm Mechanically stable Cantilever Length µm Appropriate reach Mean Width µm Mechanically stable Half Cone Angle 25 Sample dependent Base Shape configurable Sample dependent Apex Shape configurable Sample dependent Resonant Frequency Coating Several khz, depends on shape None, Gold, Platinum, Diamond Matching piezo s resonant frequency Experiment dependent

55 Details of Parts of AFM Shapes of AFM Tip

56 Details of Parts of AFM Shapes of AFM Tip Protruding from the Very End Positioned at the Very End Square-Based Pyramid Rectangularbased Pyramid Circular Symmetric Spike

57 Details of Parts of AFM High Aspect Ratio Spike AFM Tips Focused Ion Beam Electron Beam Deposited Carbon Nanotube Plateau Rounded Sphere Critical Dimension

58 Details of Parts of AFM Scanner In most AFMs piezoelectric materials are used to achieve this. These change dimensions with an applied voltage. The diagram below shows a typical scanner arrangement.

59 Details of Parts of AFM Scanner Equivalent Circuit Model The presence of electrical resonances and anti-resonances make the piezoelectric impedance unique. The resonances result from the electrical input signal exciting a mechanical resonance in the piezo element.

60 Details of Parts of AFM Feedback The feedback system is affected by three main parameters: 1. Setpoint 2. Feedback gains 3. Scan rate

61 Optical AFM Advanced Surface Topography technique avoids cantilever mechanism by use of optical fiber based tips and using Fabry Pérot Interferometry (or Etalon): There is only one limitation of such an approach: surface of the sample should be smooth enough and homogeneously reflecting.

62 Artefacts in AFM Scanner Related Hysteresis The piezoelectric s response to an applied voltage is not linear. This gives rise to hysteresis.

63 Artefacts in AFM Scanner Related Scanner creep If the applied voltage suddenly changes, then the piezo-scanner s response is not all at once. It moves the majority of the distance quickly, then the last part of the movement is slower. This slow movement will cause distortion, known as creep. Change in x-offset Change in y-offset Change in size

64 Artefacts in AFM Scanner Related Bow and Tilt Because of the construction of the piezo-scanner, the tip does not move in a perfectly flat plane. Instead its movement is in a parabolic arc (scanner bow). Also the scanner and sample planes may not be perfectly parallel (tilt). Both of these artefacts can be removed by using post-processing software.

65 Blunt tip: Use Feedback Mode Artefacts in AFM Tip Related Tip picks up debris: Cleaning the sample with compressed air or N 2 before use

66 Artefacts in AFM Feedback Related Poor tracking due to high scan rate Gains are set too high, then the feedback circuit can begin to oscillate. This causes high frequency noise

67 Artefacts in AFM Vibration Related AFMs are very sensitive to external mechanical vibrations, which generally show up as horizontal bands in the image. These can be minimised by the use of a vibrational isolation table, and locating the AFM on a ground floor or below. Acoustic noise such as people talking can also cause image artefacts, as can drafts of air. An acoustic hood can be used to minimise the effects of both of these.

68 Beyond just surface Seeing the atomic orbital

69 Beyond just surface Seeing the atomic orbital Ref: Minghuang Huang, Martin Cuma, and Feng Liu. (27 June, 2003). Seeing the Atomic Orbital: First- Principles Study of the Effect of Tip Termination on Atomic Force Microscopy. Physical Review Letters. Volume 90, Number 25.

70 Beyond just surface Seeing the reaction Work done by Franz J. Giessibl at the Department of Physics, University of Regensburg have been success to image chemical reaction using AFM by having a carbon monoxide molecule at the tip to obtain high spatial resolution.