Microscopie a stilo: principi ed esempi di applicazione

Transcription

1 Microscopie a stilo: principi ed esempi di applicazione Adele Sassella Dipartimento di Scienza dei Materiali Università degli Studi di Milano Bicocca adele.sassella@unimib.it Pavia, 22 aprile 2009

2 SCANNING PROBE MICROSCOPIES - SPM detection of: topography, electronic state density, magnetic, electric, elastic, thermal, optical properties, friction, doping, BY local sample-probe interaction

3 SCANNING PROBE MICROSCOPIES - SPM FORCE AND INTERACTION TECHNIQUE tunnel current van der Waals force+repulsion friction electric force capacitance magnetic force light transmission, reflection, and emission heat transfer STM, STS AFM LFM EFM SCM MFM SNOM SThM

4 COMMON CHARACTERISTICS - 1 a microscopic probe scans the sample surface at very short distance (< 50 nm) with high spatial resolution the local interaction between sample and probe is made detectable by a proper transducer a map of the specific sample property is collected fast scan direction (i) positions for data collection scan direction, slow scan direction (j) and tip recovery

5 COMMON CHARACTERISTICS - 2 a piezoelectric scanner is used for finely tuning the sample-probe relative position: l/l = d E l length, E electric field, d strain coefficient lateral resolution ~ 1 Å vertical resolution ~ 0.01 Å sample sample tripode scanner: independent x, y, z motion tubular scanner: dependence of the motion along the three axes

6 COMMON CHARACTERISTICS - 3 the 2D map of the signal (current, height, light intensity, temperature, ) needs image treatments for STM and high resolution AFM: Fourier analysis + selection of the frequencies + Fourier anti-transform FT antift 4T/RUB heteroepitaxy

7 the first one: SCANNING TUNNELING MICROSCOPY - STM G. Binning and H. Rohrer - Nobel laureates in Physics in 1986 "for their design of the scanning tunneling microscope" Helv. Phys. Acta 55, 726 (1982) basis phenomenon: tunnel current need for conducting samples (metals or semiconductors) vacuum, controlled atmosphere sample-probe local interaction atomic resolution

8 EXP APPARATUS INTERACTION Φ (z) tip sample I tunnel Vpol potential barrier x-y-z scanner z W tip under proper conditions a tunnel current can flow

9 SAMPLE-TIP INTERACTION: TUNNEL EFFECT potential barrier Φ (z) Φ 0 -b b z z < -b tip -b < z < b gap (air, vacuum, ) z > b sample average work function Φ 0 classical mechanics particles with kinetics energy W lower than Φ 0 do not cross the barrier τ = 0 and ρ = 1 quantum mechanics particles with kinetics energy W lower than Φ 0 can cross the barrier τ ρ = 1 + = Φ0senh 4W W kb 0 ( Φ ) ( Φ ) 4W W Φ senh 2 0 2kb 1 1 and

10 METALLIC TIP and SAMPLE (limit of STM!) => particles are electrons => TUNNEL CURRENT CAN FLOW current density (electrons from tip to sample) J E D t (E) f t (E) D s (E) [ 1 f ( E) ] τ de s E D t D s f t,s (E) τ energy tip electron state density sample electron state density Fermi-Dirac distribution function tunnel probability at equilibrium, f s (E) = f t (E) and current flows in the two directions => can be collected no current

11 when a voltage is applied between tip and sample, the barrier gets distorted and f is centered on different energy levels potential barrier + applied voltage Φ (z) Φ 0 E F E F -ev pol -b b z tunnel current flows in one direction only => J tunnel 0 and tunnel current can be collected

12 therefore J tunnel = e h 2 A Φ 2πd 0 V pol exp ( A Φ d ) 0 d tip-sample distance; 2m A= 2 h 1/2 1 = ev nm 2 the exponential dependence of J on d gives STM atomic resolution for d =d+1å: J tunnel (d ) ~ 0.1 J tunnel (d) tip V pol 99% current d 90% current sample

13 What is it an STM image? It is a map of the current values, collected during sample scan What does it represent? It is the map of the surface electronic state density of the sample; only in a very rough approximation, often misleading, it can be interpreted as a topography GRAPHITE 2.46 Å carbon atoms

14 vacuum or controlled atmosphere WORKING SCHEME i V cantilever controller computer tip x-y sample (feedback) (Fourier analysis) scanner z STM image microscope TYPICAL OPERATING CONDITIONS: V pol 1V sample-tip distance 0.7 nm I tunnel ~ na 3D view of the Ni(110) surface from STM analysis

15 OPERATING MODES: 1- constant current mode the sample is scanned maintaining the sample-tip distance constant the sample position is monitored and tuned by an electronic feedback circuit driving the scanner the image is drawn using the values of the voltage supplied by the feedback circuit tip path tunnel current sample profile + high dynamic range - slow imaging (limited by electronics) + high linearity

16 OPERATING MODES: 2- constant height mode the sample is scanned maintaining the height of the cantilever constant (with respect to the laboratory frame) the sample-tip distance is varying during scanning due to the sample morphology, so that their interaction varies the tunnel current intensity changes during scan the image is drawn using directly the current signal tip path tunnel current sample profile + fast imaging - limited dynamic range + high resolution - small and flat sample regions - possible tip crash

17 EXAMPLES 30 nm x 30 nm oxygen-induced facetting of vicinal Cu(100): a region surrounding a ridge between two different facets of {410} type

18 8 nm x 8 nm reconstruction of Si(111) surface

19 50 nm x 50 nm GaN(0001) surface, where three characteristics features are visible: molecular steps (2.6 Å in height) and holes marked e and m (edge and mixed dislocations, i.e. with a screw component)

20 8 nm x 8 nm GaN(0001)(4x4) surface made by deposition of 1,5 monolayer (Ga+N) of gallium on a (2x2) nitrogen rich surface.

21 ATOM MANIPULATION WITH STM: LITHOGRAPHY Three main methods permit the use of STM for manipulating atoms and molecules, based on: - TRANSFER OF ENERGY (HEAT) - ENERGY BARRIER VARIATION - USE OF ELECTRON-RESIST MATERIALS lithography has the opposite goal when compared to the common imaging use of STM

22 building a circle of Fe atoms on Cu(111) surface 1 nm electron wave functions = standing waves

23 elaborating the images No topography!

24 ATOMIC FORCE MICROSCOPY AFM

25 G. Binning, F.C. Quate, and Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986) no need for conducting samples vacuum, controlled atmosphere, liquid ambient sample-probe manybody interaction molecular resolution atomic resolution under specific conditions sample-probe interaction: attractive and repulsive forces

26 INTERACTIONS IN AFM van der Waals (dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole) Pauli repulsion nuclear repulsion physi- and chemi- sorption adhesion in metals friction capillarity elasticity plastic deformation R tip contamination layer R 50 nm

27 TYPICAL PYRAMID Si 3 N 4 TIP 5 µm 100 µm

28 SAMPLE-TIP INTERACTION forces described by the Lennard-Jones potential elastic force of the cantilever capillary force 1. CONTACT MODE cantilever deflection surface morphology CONTACT 2. NON-CONTACT MODE NON-CONTACT amplitude of the cantilever oscillation surface morphology

29 TOTAL INTERACTION: LENNARD-JONES POTENTIAL V LJ = C r 12 C' 6 r = 4ε σ r 12 σ r 6 ε depth of the minimum σ distance r where V=0 (also called r 0 ) r e = 2 1/6 σ equilibrium distance, where V=min

30 laser position sensitive photodetector WORKING SCHEME cantilever scanner tip x-y z sample feedback controller computer AFM image microscope slow fast scan path 4T homoepitaxy

31 POSITION SENSITIVE PHOTODETECTOR four-section detector (photodiode) 1 2 laser light spot 3 4 from the different sections the photocurrent is collected: I 1, I 2, I 3, I 4 VERTICAL movements of the cantilever are detected by: (I 1 +I 2 ) - (I 3 +I 4 ) HORIZONTAL movements of the cantilever are detected by: (I 1 +I 3 ) - (I 2 +I 4 ) 10 Å movement of the light spot can be detected, corresponding to 0.5 Å movement of the cantilever

32 ATOMIC RESOLUTION given the r -n dependence of the potential, a many body interaction drives AFM operation (remind the exponential dependence of tunnel current in STM, giving rise to atiomic resolution) NONETHELESS, atomic resolution is achievable scan TIP SAMPLE atoms 2 and 3 atom 1 signal atom 1: vacancy in position 7, high intensity signal atom 2: vacancy in position 8, lower intensity signal atom 3: vacancy in position 6, lower intensity TOTAL SIGNAL: + ATOMIC RESOLUTION -NO VACANCY

33 epitaxial 4T films on 4T bulk crystal anthracene films on KAP 5 5 µm 2 prediction high resolution 9 9 nm 2 image 5 5 nm 2

34 4T on rubrene nm 2 images µm 2 image FT transform epitaxial relations 4T(001)//RUB(100) 4T(110)//RUB(021)

35 NaCl(100) su mica 4 4 nm 2

36 OPERATING MODES: 1. CONTACT MODE, REPULSIVE POTENTIAL cantilever dynamics L (side view) (top view) h F s b s 3 4L = E bh 3 F E - Young modulus (E = Nm -2 for W, E = Nm -2 for Si 3 N 4 ) for a typical Si 3 N 4 cantilever for AFM: L = 180 µm, b = 20 µm, h = 0.6 µm, so that: s ~ 27 F ( s in m, F in N) when s = 1 Å, F ~ N

37 1. CONTACT MODE active forces: repulsive force (F R ~ 10-7 N) capillary force (F C ~ 10-8 N) elastic force (F E ~ 10-7 N) F C F E F R at equilibrium: F C + F E = F R ~ k s since F C << F E here k ~ Nm -1 is the cantilever elastic constant How large can k be? Let s compare it with the effective elastic constant k eff originating from atom aggregation in solids: f vib ~ Hz = 2π and µ ~ kg k 1 eff µ k eff ~ 1 10 Nm -1 for k (cantilever) of few Nm -1 the sample surface can be damaged

38 CONSTANT HEIGHT MODE 1. CONTACT MODE the sample is scanned maintaining the height of the cantilever constant (with respect to the laboratory frame) the sample-tip distance is varying during scanning due to the sample morphology, so that their interaction varies the cantilever gets deflected, giving a signal difference to the photodetector the image is drawn using signal of the photodetector tip path force intensity sample profile - the detector is sensitive to a cantilever s of less than 1 Å - for a cantilever k ~ Nm -1 F = k s ~ N can be measured + fast imaging - limited dynamic range + high resolution - small and flat sample regions - possible tip crash

39 CONSTANT FORCE MODE 1. CONTACT MODE the sample is scanned maintaining the sample-tip distance constant the sample position is monitored and tuned by an electronic feedback circuit driving the scanner the image is drawn using the voltage supplied by the feedback circuit tip path force intensity sample profile + high dynamic range - slower imaging (limited by electronics) + high linearity - small and flat sample regions - possible tip crash

40 OPERATING MODES: 2. NON-CONTACT MODE, ATTRACTIVE POTENTIAL Å amplitude useful for: elastic surfaces easily damageable surfaces Å distance useless for: contaminated surfaces (water) oscillation frequency of the free cantilever: k is the elastic constant m ~0.2 m is the effective mass f n 1 = 2π k m ' during oscillation, the VdW interaction between sample and tip makes the cantilever elastic constant change by k and a new effective elastic constant k eff is to be considered

41 2. NON-CONTACT MODE k' F z VdW = grad z FVdW = V LJ ~ z -6 then F VdW ~ z -7 and grad z F VdW ~ z -8 k eff = k k and f ' n 1 = 2π k eff m' the sample-tip distance decreases the force gradient increases f n decreases detecting the variation of the cantilever frequency (and amplitude) permits drawing a map of the tip-sample interaction directly related to the sample TOPOGRAPHY

42 OPERATING MODES: 3. INTERMITTENT-CONTACT MODE, ATTRACTIVE + REPULSIVE POTENTIAL Å amplitude Å distance useful for: elastic surfaces easily damageable surfaces + contaminated surface (water) as in the non-contact mode, the sample-tip distance decreases the force gradient increases f n decreases

43 3. INTERMITTENT-CONTACT MODE detecting the variation of the cantilever frequency (and amplitude) permits drawing a map of the tip-sample interaction directly related to the sample TOPOGRAPHY images are free from articfacts coming from surface contamination non-contact profile sample + water drop intermittent contact profile

44 6T films grown on KAP (010) by OMBE: homoepitaxy 5 5 µm 2 AFM images of the surface of α6t submonolayer homoepitaxial films grown on α6t/lt(100) under the same conditions at a) 298 K (25 C), b) 333 K (60 C), c) 363 K (90 C), and d) 393 K (120 C)

45 4T films grown on KAP (010) by MBE µm 2 images a) 10 nm thick film (height image) b) 80 nm thick film (error signal) scheme of the structure

46 17 17 µm 2 images Tetraphenylporphyrin molecules deposited on functionalized quartz

47 cromosomes adsorbed on a solid surface

48 LATERAL FORCE MICROSCOPY - LFM a) b) c) front view tip sample lateral force profile light spot on the PSPD a) b) c) (I 1 +I 3 ) - (I 2 +I 4 ) = 0 (I 1 +I 3 ) - (I 2 +I 4 ) > 0 (I 1 +I 3 ) - (I 2 +I 4 ) < 0

49 ORGANIC EPITAXY: 4T on KAP µm 2 image of a 2 nm thick 4T film (LFM image) 5 5 µm 2 image of a 10 nm thick 4T film (LFM image)

50 SUMMARY AFM no need for conducting samples vacuum, controlled atmosphere, liquid ambient sample-probe manybody interaction molecular resolution atomic resolution under specific conditions sample-probe interaction: attractive and repulsive forces

51 Manipulating atoms and molecules by AFM: 1. NANOINDENTATION the surface of a sample is displaced as pressure is applied by the tip of an AFM probe. The "Force-Displacement" dependence provides the hardness of the sample at a given point Loading-unloading curves. h - displacement, P - load, S - contact stiffness. 5 µm x 5 µm scan on a sapphire surface with indents: topography.

52 2. NANOSCRATCHING making scratches on the sample surface and measuring their parameters, i.e. depth and especially width, gives an opportunity to evaluate the hardness of materials quantitatively. the width of a scratch, as the result of the elastic recovery, may modifies less than its depth; deeper information can be obtained ith respect to the static nanoindentation. 4 µm x 4 µm three scratches of different depths on a fused quartz surface, observed by AFM. depth & width of the three scratches

53 3. DIP-PEN NANOLITHOGRAPHY R.D. Piner, J. Zhu, F. Xu, S. Hong, and C.A. Mirkin, Science 283, 661 (1999) in air, water condenses in the gap between tip and sample depending on the relative humidity and on the sample wetting properties, a thin water layer can be formed with nm-scale resolution when properly chosen molecules are on the tip, they can be transferred and anchored onto the sample surface

54 2- reading by LFM (friction imaging) - tip velocity: 10 µm/s 1- writing by DPN (contact imaging) - relative humidity: 5% - tip velocity: 1 2 µm/s

55 Octadecanethiols on Au thin films µm µm 2 AFM friction image µm 2 3D image of the µm 2 scan

56 8 8 µm 2 AFM friction images: arrays of 1 1 µm 2 squares tetrafluoro-thiomethyl acridine on Au thin films

57 DPN logo - P. Campiglio, 2008 Microscopie a stilo: principi ed esempi di applicazione Adele Sassella Dipartimento di Scienza dei Materiali - Università degli Studi di Milano Bicocca adele.sassella@unimib.it Pavia, 22 aprile 2009