(Nano)materials characterization MTX9100 Nanomaterials Lecture 8 OUTLINE 1 -What SEM and AFM are good for? - What is the Atomic Force Microscopes Contribution to Nanotechnology? - What is Spectroscopy?
Atomic Force Microscopes (AFM) The Atomic Force Microscope was developed to overcome a basic drawback with STM - that it can only image conducting or semiconducting surfaces. The AFM, however, has the advantage of imaging almost any type of surface, including polymers, ceramics, composites, glass, and biological samples. 2
Why AFM? An atomic force microscope (AFM) creates a highly magnified three dimensional image of a surface. The magnified image is generated by monitoring the motion of an atomically sharp probe as it is scanned across a surface. With the AFM it is possible to directly view features on a surface having a few nanometer-sized dimensions including single atoms and molecules on a surface. This gives scientists and engineers an ability to directly visualize nanometer-sized objects and to measure the dimensions of the surface features. 3
Atomic Force Microscopes Monitors the forces of attraction and repulsion between a probe and a sample surface The tip is attached to a cantilever which moves up and down in response to forces of attraction or repulsion with the sample surface Movement of the cantilever is detected by a laser and photodetector ZnO Shading shows interaction strength STM AFM 4 Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer, where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. AFM tips and cantilevers are microfabricated from Si or Si 3 N 4. Typical tip radius is from a few to 10s of nm.
Schematic of the AFM 5 operation Because the atomic force microscope relies on the forces between the tip and sample, knowing these forces is important for proper imaging. The force is not measured directly, but calculated by measuring the deflection of the lever, and knowing the stiffness of the cantilever. Hook s law gives F = -kz, where F is the force, k is the stiffness of the lever, and z is the distance the lever is bent.
Measuring forces The fundamental interaction at short distances is the van der Waals interactions, which are responsible for the formation of solids, wetting, etc. At distances of a few nm, van der Waals forces are sufficiently strong to move macroscopic objects such as AFM cantilevers. Van derwaals interactions consist of three components: polarization, induction, and dispersion. Polarization refers to permanent dipole moments such as exist in water molecules. Induction refers to the contribution of induced dipoles. Dispersion is due to instantaneous fluctuations of electrons, which occur at 6 the frequency of light.
Modes of operation - Used for Contact Mode, Non-contact and TappingMode AFM -Laser light from a solid state diode is reflected off the back of the cantilever and collected by a position sensitive detector (PSD). This consists of two closely spaced photodiodes. The output is then collected by a differential amplifier - Angular displacement of the cantilever results in one photodiode collecting more light than the other. The resulting output signal is proportional to the deflection of the cantilever. - Detects cantilever deflection <1A 7
Contact mode Constant force is applied to the surface while scanning Potential diagram showing the region of the probe while scanning in contact mode. In contact mode the probe glides over the surface. 8 Contact mode is typically used for scanning hard samples and when a resolution of greater than 50 nanometers is required. The cantilevers used for contact mode may be constructed from silicon or silicon nitride. Resonant frequencies of contact mode cantilevers are typically around 50 KHz and the force constants are below 1 N/m.
Contact mode images 9 Left: Bits on a compact disk. Center: Image of a metal surface. Right: Nano-particles on a surface - A tip is scanned across the sample while a feedback loop maintains a constant cantilever deflection (and force) - The tip contacts the surface through the adsorbed fluid layer. - Forces range from nano to micro N in ambient conditions and even lower (0.1 nn or less) in liquids. http://www.afmuniversity.org/chapter4_5.html
Tapping mode The probe is vibrated in and out of surface potential. The modulated signal can then be processed with a phase or amplitude demodulator. -A cantilever with attached tip is oscillated at its resonant frequency and scanned across the sample surface. - A constant oscillation amplitude (and thus a constant tip-sample interaction) are maintained during scanning. Typical amplitudes are 20-100nm. - Forces can be 200 pn or less - The amplitude of the oscillations changes when the tip scans over bumps or 10depressions on a surface.
Tapping mode images Vibrating mode AFM images. Left: Silicon wafer. Center: Cancer cells. Right: Proteins. AFM cantilever 11 http://www.afmuniversity.org/chapter4_5.html
Non contact mode -The cantilever is oscillated slightly above its resonant frequency. - Oscillations <10nm -The tip does not touch the sample. Instead, it oscillates above the adsorbed fluid layer. - A constant oscillation amplitude is maintained. -The resonant frequency of the cantilever is decreased by the van derwaals forces which extend from 1-10nm above the adsorbed fluid layer. This in turn changes the amplitude of oscillation. In contact mode, the tip is usually maintained at a constant force by moving the cantilever up and down as it scans. In non-contact mode or tapping mode the tip is driven up and down by an oscillator. Especially soft materials may be imaged by a magnetically-driven cantilever (MAC ModeTM). In non-contact mode, the bottom-most point of each probe cycle is in the attractive region of the force-distance curve. In tapping and contact mode the bottom-most point is in the repulsive region. Variations in the measured oscillation amplitude and phase in relation to the driver frequency are indicators of the surfaceprobe interaction. 12
Advantages of the main modes of AFM 13 Contact Mode Tapping Mode High scan speeds The only mode that can obtain atomic resolution images Rough samples with extreme changes in topography can be scanned more easily 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 of the main modes of AFM Contact Mode Tapping Mode 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) Slightly lower scan speed than contact mode AFM 14
15 SPM family
SPM AFM tips 16 There are several types of Scanning Probe Microscopes that are used for measuring surface topography and physical properties. The primary types of SPM's are the AFM, STM and NSOM. AFM's account for about 80% of the total number of scanning probe microscopes.
What SEM and AFM are good for? 17 Both the AFM and SEM measure topography. However, both types of microscopes can measure other surface physical properties. The SEM is good for measuring chemical composition and the AFM is good for measuring mechanical properties of surfaces. http://www.afmuniversity.org/chapter1_8.html
Manipulating materials at the atomic level A tip is used to pick up atoms by attracting them with an electrostatic field. This tip is moved to the predefined position and the atom released by turning off the electric field. 18
Phase imaging Accessible via Tapping Mode 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. 19 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
Phase Imaging - examples Composite polymer imbedded in a matrix MoO3 crystallites on a MoS2 substrate 20 Image/photo taken with NanoScope SPM, courtesy Digital Instruments, Santa Barbara,CA
Nanoindentation and Scratching» A diamond tip is mounted on a metal cantilever and scanned either with contact or Tapping Mode.» Indenting mode presses the tip into the sample» Scratch mode drags the tip across the sample at a specific rate and with a specified force.» The use of Tapping Mode makes it possible to simultaneously image soft samples 21
Nanoindenter Indentation cantilevers are thicker, wider, and longer than standard AFM cantilevers and are diamonds, as compared with silicon or silicon nitride. The typical ranges for spring constants of contact mode, Tapping Mode, and indentation cantilevers are 0.01-1.0N/m, 20-100N/m, and 100-300N/m respectively. The resonant frequency for indentation cantilevers is generally in the range of 35-60kHz, depending on the dimensions of the cantilever and the size of the diamond. For comparison, the resonant frequency for standard Tapping Mode cantilevers is about 300kHz. 22
Nanoindentation - basic S = dp dh 23 For Berkovich tip: A = 24.5h c 2, where h c is the contact depth h c = h max - 0.75(r max /s).
Analytical model Basic concept Deformation upon unloading is purely elastic The compliance of the sample and of the indenter tip can be combined as springs in series Hardness 1 E * = 2 1 υ 1 υ ' + E E' 2 24 H = P 24.5hp 2
Comparison of AFM and other imaging techniques 1. AFM versus STM: In some cases, the resolution of STM is better than AFM because of the exponential dependence of the tunneling current on distance. The force-distance dependence in AFM is much more complex when characteristics such as tip shape and contact force are considered. STM is generally applicable only to conducting samples while AFM is applied to both conductors and insulators. In terms of versatility, the AFM wins. Furthermore, the AFM offers the advantage that the writing voltage and tip-tosubstrate spacing can be controlled independently, whereas with STM the two parameters are integrally linked. 2. AFM versus SEM: Compared with Scanning Electron Microscope, AFM provides extraordinary topographic contrast direct height measurements and unobscured views of surface features (no coating is necessary). 3. AFM versus TEM: Three dimensional AFM images are obtained without expensive sample preparation and yield far more complete information than the two dimensional profiles available from cross-sectioned samples. 4. AFM versus Optical Microscope: The AFM provides unambiguous measurement of step heights, independent of reflectivity differences between materials. 25
Comparison of imaging techniques 26 An AFM is capable of resolving features in the dimensions of a few nanometers with scan ranges up to a hundred microns.
Summary of tools 27 Comparison of the time for measurements and instrumentation cost of optical, AFM, and SEM/TEM microscopes.
Spectroscopy Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength (λ). The sample is irradiated with an electron probe. The incident electron beam causes ionization of electrons belonging to the inner shells of the atoms composing the material. 28
What is light? Light consists of oscillating electric and magnetic fields. Because nuclei and electrons are charged particles, their motions in atoms and molecules generate oscillating electric fields. 29
Light matter interaction E total = E spin + E translation + E rotation + E vibration + E electrons + E nucleus The total energy of a molecule 1913 - Bohr postulated that a quantum of light of angular frequency ω is absorbed or emitted whenever an atom jumps between two quantized energy levels E 1 and E 2 E 2 E 1 = ħω 1916 7 Einstein introduced the Einstein coefficients to quantify the rate at which the absorption and emission of quanta occur 30
What happens when light interacts with a molecule? Absorption Emission Scattering 31 A transition from a lower level to a higher level with transfer of energy from the radiation field to an absorber, atom, molecule, or solid. A transition from a higher level to a lower level with transfer of energy from the emitter to the radiation field. If no radiation is emitted, the transition from higher to lower energy levels is called nonradiative decay. Redirection of light due to its interaction with matter. Scattering might or might not occur with a transfer of energy, i.e., the scattered radiation might or might not have a slightly different wavelength compared to the light incident on the sample.
Radiation Type of Radiation Frequency Range (Hz) Wavelength Range Type of Transition gamma-rays 10 20-10 24 <1 pm nuclear X-rays 10 17-10 20 1 nm-1 pm inner electron ultraviolet 10 15-10 17 400 nm-1 nm outer electron visible 4-7.5x10 14 750 nm-400 nm outer electron near-infrared 1x10 14-4x10 14 2.5 µm-750 nm outer electron molecular vibrations infrared 10 13-10 14 25 µm-2.5 µm molecular vibrations microwaves 3x10 11-10 13 1 mm-25 µm molecular rotations, electron spin flips* radio 32 waves <3x10 11 >1 mm nuclear spin flips*
Classification of methods The type of spectroscopy depends on the physical quantity measured. Normally, the quantity that is measured is an intensity, either of energy absorbed or produced. Electromagnetic spectroscopy involves interactions of matter with electromagnetic radiation, such as light. Electron spectroscopy involves interactions with electron beams. Auger spectroscopy involves inducing the Auger effect with an electron beam. In this case the measurement typically involves the kinetic energy of the electron as variable. Mass spectrometry involves the interaction of charged species with magnetic and/or electric fields, giving rise to a mass spectrum. The term "mass spectroscopy" is deprecated, for the technique is primarily a form of measurement, though it does produce a spectrum for observation. This spectrum has the mass m as variable, but the measurement is essentially one of the kinetic energy of the particle. Acoustic spectroscopy involves the frequency of sound. Dielectric spectroscopy involves the frequency of an external electrical field Mechanical spectroscopy involves the frequency of an external mechanical 33 stress, e.g. a torsion applied to a piece of material.
Absorption spectroscopy Absorption spectroscopy refers to a range of techniques employing the interaction of electromagnetic radiation with matter. The intensity of a beam of light measured before and after interaction with a sample is compared. By measuring the frequencies of light absorbed by an atom or molecule, we can determine the frequencies of the various transition motions that the atom or molecule can have. Thus we can use light absorption to probe the dynamics of atoms and molecules! An atom or molecule can absorb energy from light if the frequency of the light oscillation and the frequency of the electron or molecular "transition motion" match. Unless these frequencies match, light absorption cannot occur. The "transition motion" frequency is related to the frequencies of motion in the higher and lower energy states. 34
Infrared spectroscopy IR spectroscopy allows to examine the vibrational motions of molecules. E translation, E rotation, E vibration 35 The quantum energy of infrared photons is in the range 0.001 to 1.7 ev which is in the range of energies separating the quantum states of molecular vibrations. Infrared is absorbed more strongly than microwaves, but less strongly than visible light. The result of infrared absorption is heating of the tissue since it increases molecular vibrational activity.
Raman spectroscopy When electromagnetic radiation passes through matter, most of the radiation continues in its original direction but a small fraction is scattered in other directions. Light that is scattered at the same wavelength as the incoming light is called Rayleigh scattering. Light that is scattered in transparent solids due to vibrations (phonons) is called Brillouin scattering. Brillouin scattering is typically shifted by 0.1 to 1 cm -1 from the incident light. Light that is scattered due to vibrations in molecules or optical phonons in solids is called Raman scattering. Raman scattered light is shifted by as much as 4000 cm -1 from the incident light. 36 Raman spectroscopy is the measurement of the wavelength and intensity of inelastically scattered light from molecules. The Raman scattered light occurs at wavelengths that are shifted from the incident light by the energies of molecular vibrations. The mechanism of Raman scattering is different from that of infrared absorption, and Raman and IR spectra provide complementary information. Typical applications are in structure determination, multicomponent qualitative analysis, and quantitative analysis.
Raman spectroscopy - basic Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other lowfrequency modes in a system. It relies on inelastic scattering of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system. IR spectroscopy yields similar, but complementary, information. 37
Basic theory of RS The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud of the bonds of that molecule. The incident photon excites one of the electrons into a virtual state. For the spontaneous Raman effect, the molecule will be excited from the ground state to a virtual energy state, and relax into a vibrational excited state, which generates Stokes Raman scattering. If the molecule was already in an elevated vibrational energy state, the Raman scattering is then called anti-stokes Raman scattering. A molecular polarizability change, or amount of deformation of the electron cloud, with respect to the vibrational coordinate is required for the molecule to exhibit the Raman effect. The amount of the polarizability change will determine the Raman scattering intensity, whereas the Raman shift is equal to the vibrational level that is involved. 38
Raman microspectroscopy Raman spectroscopy offers several advantages for microscopic analysis. Since it is a scattering technique, specimens do not need to be fixed or sectioned. Raman spectra can be collected from a very small volume (< 1 µm in diameter); these spectra allow the identification of species present in that volume. Water does not interfere very strongly. Raman spectroscopy is suitable for the microscopic examination of minerals, materials such as polymers and ceramics, cells and proteins. A Raman microscope begins with a standard optical microscope, and adds an excitation laser, a monochromator, and a sensitive detector (such as a charge-coupled device (CCD), or photomultiplier tube (PMT)). FT- Raman has also been used with microscopes. 39
40 Characterization techniques