Graphene and 2D Materials & Devices PDF - Characterization Techniques

Graphene and 2D Materials & Devices Course Instructor Dr George Kakavelakis ([email protected]) Assistant Professor of Printed Nanoelectronics Department of Electronic Engineering Hellenic Mediterranean University How to identify an...

Graphene and 2D Materials & Devices Course Instructor Dr George Kakavelakis ([email protected]) Assistant Professor of Printed Nanoelectronics Department of Electronic Engineering Hellenic Mediterranean University How to identify an unknown sample? Characterization techniques for layered materials  Outline Part I: Surface Characterization Part II: Structural Characterization Part III: Spectroscopic Characterization Part IV: Electrical Characterization of nanomaterials films Part I: Surface Characterization  Part I: Surface Characterization Optical Microscopy ✓ An optical microscope is an optical system used for the visual observation of objects under magnification with the help of light. ✓ The observation can be made either through the reflected or through the transmitted light, from the objects to be observed.  Part I: Surface Characterization Optical Microscopy Optical images of steel’s microstructure  Part I: Surface Characterization Optical Microscopy Optical images of exfoliated flakes  Part I: Surface Characterization Scanning Electron Microscopy ✓ The capability of optical microscopes is limited by the nature of light to magnification levels up to 1000x and resolution down to 0.2 µm. ✓ A scanning electron microscope is an instrument that works much like an optical microscope except that it uses a high-energy electron beam instead of light to examine objects on a detailed scale.  Part I: Surface Characterization Scanning Electron Microscopy The basic stages of operation of an electron microscope are: ✓ A beam of electrons is formed from the source which is accelerated by a positive electric potential. ✓ Using metal apertures, electromagnetic lenses and scanning coils, a narrowly focused monochromatic beam is achieved which scans the surface of the sample. ✓ The sample-beam interactions are recorded by the detectors and converted into an image.  Part I: Surface Characterization Scanning Electron Microscopy ✓ From this interaction information is obtained in relation to the atoms of the elements that make up the examined material. ✓ The intensity of the emitted electrons is affected by the characteristics of the surface. ✓ Can reach a spatial resolution up to 0.4 nm with some models and lenses SEM images: a) Structure of anti-reflective surface produced by self-assembly, b) ZnO nanowire structures. Chen H et al, Nanoscale Res Lett (2009)  Part I: Surface Characterization Scanning Electron Microscopy SEM images of exfoliated graphene flakes  Part I: Surface Characterization Scanning Electron Microscopy The difference in resolution of OM (left) Vs SEM (right) images  Part I: Surface Characterization Scanning Tunneling Microscopy ✓ The operating principle of the STM is based on the principles of quantum mechanics. If we take two hydrogen atoms (A and B) next to each other, then the electron of A has some probability (not a large one) of being, according to the principles of quantum mechanics, in the space of B. That is, in other words, electrons can flow through "tunneling" from one atom to another. ✓ The scanning tunneling microscope (STM), as shown in the next figure, consists of a tungsten probe that is placed extremely close to the sample of atoms we want to image. ✓ In 1986 the Nobel Prize in Physics was awarded for the development of STM by Gerd Binnig and Heinrich Rohrer in the early 1980s at IBM Research - Zurich  Part I: Surface Characterization Scanning Tunneling Microscopy ✓ This spike tip, which is 1-2 atoms, accurately approaches the sample within an atomic radius (1Å). The electrons then flow from the tip to the sample or vice versa. ✓ The spike then scans the surface of the sample, moving up- down, right-left, to keep the current constant. In this way the tip follows the outline of the curves of the atoms in the sample. ✓ These movements are controlled by a computer, which, processing the data, finally gives the topographic representation of the atoms, with "valleys" and "hills", on the surface of the sample.  Part I: Surface Characterization Scanning Tunneling Microscopy STM image of (left) HOPG and (right) Pt surface, vacancies are clearly visible  Part I: Surface Characterization Scanning Tunneling Microscopy Atomic resolution with STM  Part I: Surface Characterization Atomic Force Microscopy ✓ The construction of the AFM was based on the Scanning Tunneling Microscope (STM), of which it was an improvement as the latter could only be used on conducting samples. ✓ A sharp tip attached to the end of a small cantilever, so thin and flexible as to resemble a spring, scans the sample, which lies beneath it. The relative movement of the sample with respect to the tip, in the xy plane but also in the vertical axis, is controlled by means of a piezoelectric scanner. ✓ Holding the tip on the surface during scanning is ensured through the forces that develop between them (electrostatic, magnetic, van der Walls or others, depending on the properties of the material) and the scanning mode, which has been chosen.  Part I: Surface Characterization Atomic Force Microscopy ✓ To control scanning, a laser beam, vertically incident and reflected from the back of the cantilever, is directed through a mirror to a quadrant photodetector. ✓ At rest the cantilever is rigid, and the reflected beam is incident on the center of the photosensor. But as the tip sweeps across the surface following its topography, it is forced to bend. This bending is monitored by an electronic system, which calculates the difference of the signal in the opposite quadrants of the photodetector and activates a feedback process to restore the signal to the original position and consequently the cantilever to its equilibrium state.  Part I: Surface Characterization Atomic Force Microscopy  Part I: Surface Characterization Atomic Force Microscopy  Part I: Surface Characterization Atomic Force Microscopy  Part I: Surface Characterization Profilometry ✓ Profilometry aims to measure and characterize a surface. There are two main categories into which profilometry is divided and they are: Contact Profilometry Non-Contact Profilometry ✓ Most of the commercial profilometers have accuracy of a few nanometers (nm) and in some cases reaches a few fractions of a nanometer (nm).  Part I: Surface Characterization Profilometry ✓ The profile is measured using a diamond needle “Stylus tip” which is essentially dragged over the sample. ✓ The vertical shifts translated with the help of a piezoelectric or a diode laser and a computer in profile. ✓ Thus a line–profile of the sample is determined, and then the process is repeated. ✓ Typical features of a Stylus are the size of the needle (diameter 2μm), the tip load (0.05mg) and the accuracy considered close to 10Å.  Part I: Surface Characterization Profilometry Part II: Structural Characterization  Part II: Structural Characterization Transmission Electron Microscopy ✓ In a conventional Transmission Electron Microscope, a thin sample is irradiated by a beam of electrons of uniform current density. ✓ The electron source consists of a tungsten filament that glows when an electric current passes through it, emitting electrons. ✓ The image is formed on a screen coated with a phosphorescent substance which is excited by the electrons falling on it after they pass through the sample. ✓ The points of the sample that are not permeable to electrons give us dark areas, whereas the permeable spots give us bright areas.  Part II: Structural Characterization Transmission Electron Microscopy ✓ Electrons are emitted from one cathode, either by thermal emission, by Schottky emission, or by field emission. ✓ Then, with the help of concentrating magnetic lenses, the area illuminated as well as the focus of the beam is controlled. ✓ After the sample, the electrons are directed with the help of focusing lenses (magnetic type), to a fluorescent screen. ✓ Due to the elastic and inelastic scattering that the electrons undergo during their interaction with the sample, we should have a very thin sample (50-100nm).  Part II: Structural Characterization Transmission Electron Microscopy J. Liu (Microsc. Microanal. 10 (2004) Juekuan Yang, Nature Nanotechnology 7, 91–95 (2012) Left: HRTEM images of single crystals of (Ce0.5Zr0.5)O2, used for catalysis. Right: HRTEM micrographs of an α-tetragonal boron nanoribbon.  Part II: Structural Characterization Transmission Electron Microscopy Silicon quantum dot Core Silicon quantum dot with amorphous shell  Part II: Structural Characterization Transmission Electron Microscopy In plane C-C bond determination in graphene Out of plane C-C distance determination in graphene  Part II: Structural Characterization X-ray diffraction ✓ Diffraction: bending of a wave passing through an obstacle or an aperture. ✓ Two adjacent wave sources produce a diffraction pattern as the waves add their amplitudes (constructive interference). ✓ From the figure we see that the two rays are reflected from the first and second plane, respectively. These rays must be in phase. The diffracted rays that are in phase must satisfy Bragg's law: nλ=2dsinθ  Part II: Structural Characterization X-ray diffraction  Part II: Structural Characterization X-ray diffraction Examples of crystallographic planes XRD patterns of undoped, CuO and Cu-doped nanocrystalline ZnO particles synthesized with different concentrations of Cu.  Part II: Structural Characterization X-ray diffraction Graphite and graphene structures XRD patterns of graphite and exfoliated graphite Part III: Spectroscopic characterization  Part III: Spectroscopic Characterization ✓ Spectroscopy is the study of the interaction of light and matter. ✓ Many types of spectroscopy rely on the ability of atoms and molecules to absorb or emit electromagnetic (EM) radiation. The absorption, emission, reflection and transmission of different forms of EM radiation is related to different types of transitions.  Part III: Spectroscopic Characterization UV-Vis Absorption Spectroscopy ✓ Absorption of monochromatic radiation by the sample in the UV region (190 -400 nm) and the visible region – Vis (400 – 780 nm) (for 10-8 s). ✓ Absorptions of electromagnetic radiation are the result of energy changes in the electronic structure of molecules. ✓ The outer electrons, by rising or falling from one orbit to another, cause absorption of energy in discrete, quantized quantities. ✓ Changes in the electromagnetic energy of the molecule of a chemical compound cause changes in its dipole moment, a change that is responsible for the interaction of the chemical molecule and radiation.  Part III: Spectroscopic Characterization UV-Vis Absorption Spectroscopy ✓ In absorption spectroscopy, a Iight beam of intensity (𝑰𝟎) is directed towards a sample. Some fractions of the incident light are reflected (𝑰𝑹), absorbed (𝑰𝑨) and transmitted (𝑰T) ✓ Energy conservation requires: 𝐼0 = 𝐼𝑅 +𝐼𝐴 + 𝐼𝑇 A spectrophotometer determines 𝐼𝑇 /𝐼0 ✓ Transmittance T: T = IT/I0 ✓ Absorbance is defined as :A=lg(𝐼0/𝐼𝑇)=lg(1/𝑇) Important: Not to be confused with ‘Absorption’!  Part III: Spectroscopic Characterization UV-Vis Absorption Spectroscopy Νόμος Lambert – Beer: A = absorbance, α = extinction coefficieent, b = length of light path C = concentration  Part III: Spectroscopic Characterization UV-Vis Absorption Spectroscopy Quantum dots with different sizes and thus bandgaps Gold nanoparticles with different sizes  Part III: Spectroscopic Characterization Raman Spectroscopy ✓ When electromagnetic radiation strikes a material medium, its photons interact with the medium's molecules, causing them to scatter in various directions. If the initial energy of the photons, Εo=hvo, is preserved after the interaction the scattering is called Rayleigh elastic scattering, while if it changes then the scattering is called inelastic Raman scattering, E=h(vo±vi). Raman Effect Inelastic scattering  Part III: Spectroscopic Characterization Raman Spectroscopy  Part III: Spectroscopic Characterization Raman Spectroscopy ✓ The Raman spectra ✓ Are characteristics of the molecule. ✓ They contain information about vibrational levels of the molecule. ✓ They have sharp peaks that allow a molecule to be identified by its spectrum.  Part III: Spectroscopic Characterization Raman Spectroscopy ✓ Raman spectroscopy is an ideal technique also for the study of stress transfer for the interface region. ✓ When voltage is applied to the material under study, we have a frequency shift due to the change in the vibrational state of the structural elements of the material. ✓ In the case of tension, the bonds between the atoms are lengthened, thus limiting their modes of vibration, resulting in the reduction of the frequencies of the respective modes of vibration. The opposite phenomenon is observed in copression with a corresponding increase in vibration frequency  Part III: Spectroscopic Characterization Raman Spectroscopy  Part III: Spectroscopic Characterization Raman Spectroscopy ✓ Interpretation of the Raman Peaks of graphene: ✓ The G and the 2D (@2700 cm-1) Raman peaks change in shape, position and relative intensity with the number of graphene layers; in single graphene layer the 2D peak is Lorentzian ✓ The Raman peaks allow to distinguish a hard amorphous carbon, from a metallic nanotube, giving a much more information that another approaches ✓ The main features are the G and D peaks which lie for all the carbons at around 1560 cm-1 and 1360 cm-1 respectively for visible excitation; the ration of G/D increases with the crystallization of the graphene ✓ The G peak is strongly related to the bond stretching of sp2 atoms in both rings and chains ✓ The D peak is related with defects ✓ In high quality graphene the ration of 2D/G is approximately four; whereas in bulk graphite this ration is 0.25 ✓ The ratio of G and 2D peaks vary with the number of layers: the relative G peak intensity is about 10% to 50% of the 2D height for monolayers, roughly equivalent in bilayers and higher than the 2D peak in few layer (>2) graphene and bulk graphite  Part III: Spectroscopic Characterization Infrared Spectroscopy ✓ Infrared spectroscopy results from absorption of radiation and transition to a higher energy level. While Raman spectroscopy differs from IR in that the information is obtained from light scattering. ✓ Symmetric stretching modes tend to be the most pronounced features in Raman spectra and asymmetric in IR spectra. ✓ When a molecule interacts with an electromagnetic field, energy is transferred from the field to the molecule when Bohr's condition is satisfied, ΔE=hν.  Part III: Spectroscopic Characterization Infrared Spectroscopy ✓ The polychromatic IR radiation emitted by the source reaches the beam splitter where it is made of a translucent material, usually KBr, and 50% of the radiation is reflected and the remaining 50% passes through the splitter. The splitter is placed at an angle of 45o, with respect to the incident beam, so that the intensities of both the reflected and the penetrating part of the beam are maximum. ✓ The beam is reflected and returned to the splitter. The second fraction of IR radiation that penetrates the separator, after traveling a distance (L+d), is reflected by the moving mirror and returns to the separator. Thus, the difference of the two optical paths is δ=2d.  Part III: Spectroscopic Characterization Infrared Spectroscopy  Part III: Spectroscopic Characterization X-ray Photo-electron Spectroscopy ✓ XPS can provide information about the surface of the samples; very important measurement in graphene & 2D materials ✓ XPS employs high energy photons (X-rays – 1 keV) that excite electrons into vacuum for detection, measuring kinetic energy of the photoelectrons in coordination with the incident photon energy  Part III: Spectroscopic Characterization X-ray Photo-electron Spectroscopy  Part III: Spectroscopic Characterization X-ray Photo-electron Spectroscopy  Part III: Spectroscopic Characterization X-ray Photo-electron Spectroscopy ✓ The X-rays will ‘knock out’ electrons from the sample; the measurment of the energies of the ejected electrons will determine the composition of the surface ✓ The XPS measurement takes place within an ultra-high vacuum chamber (pressures lower than 10-9 Torr) ✓ XPS measures how much energy do we need to remove an electron from the sample, known as binding energy; the latter comes as a very distinctive characteristic of each element  Part III: Spectroscopic Characterization X-ray Photo-electron Spectroscopy ✓ The X – rays have a very deep penetration depth; the released electrons very deep within the sample will be trapped. However, electrons within the first 10 nm from the surface have higher chances to escape and be detected ✓ The area below each peak represents the number of atoms exist into investigated sample; zooming to each resolved peak can provide extra information ✓ Very useful measurement with doping or functionalised graphene or how well the reduction of graphene oxide has been performed  Part III: Spectroscopic Characterization Photoluminescence Spectroscopy Instrumentation: ✓ The basic instrument is a fluorescence spectrometer. ✓ It typically contains a light source, two monochromators, a sample chamber, and a photomultiplier tube (PMT). ✓ Two monochromators are needed, one for the selection of the excitation wavelength, a second one for the analysis of the emitted light. ✓ The detector (here photomultiplier tube) ideally detects Scheme of a fluorescence spectrometer photons with equal efficiency for all wavelengths.  Part III: Spectroscopic Characterization Photoluminescence Spectroscopy  Part III: Spectroscopic Characterization Photoluminescence Spectroscopy  Part III: Spectroscopic Characterization Time-Resolved Photoluminescence Spectroscopy ✓ TRPL is the tool of choice for studying fast electronic deactivation processes that result in the emission of photons, a process called fluorescence. The lifetime of a molecule in its lowest excited singlet state usually ranges from a few picoseconds up to nanoseconds. ✓ TRPL probes those excited states which have a radiative decay channel. ✓ A pulsed laser (such as Nd:YAG) with an ultra short (ns) pulse duration is used as an excitation source. This output can be converted by an optical parametric oscillator in a wavelength range of 210-2500nm to stimulate the sample. The resulting emission is spectrally resolved using a spectrograph, and detected by a gated intensified CCD camera. By a sequential shift of the gate window with respect to the excitation (over a range from 1ns up to several ms), it is possible to measure the spectrally resolved decay of the photoluminescence, providing information about the excited state.  Part III: Spectroscopic Characterization Time-Resolved Photoluminescence Spectroscopy Setup with steak camera (for samples with ps lifetimes) ✓ In this setup the PL signal is also spectrally resolved by a spectrograph before it hits the streak cathode. The electrons released by the streak unit travel through a time dependent field which is synchronized with the laser excitation. This causes the electrons to reach the target phosphor screen at a distinct Setup with ICCD camera (samples with ns lifetimes) height which depends upon the time after excitation that they were released in the streak cathode. The flash of striking electrons is recorded by a CCD camera. The result is a spectrally and time resolved image with a time resolution of around 15 ps.  Part III: Spectroscopic Characterization Time-Resolved Photoluminescence Spectroscopy  Part III: Spectroscopic Characterization Time-Resolved Photoluminescence Spectroscopy  Part III: Spectroscopic Characterization Time-Resolved Photoluminescence Spectroscopy  Part III: Spectroscopic Characterization Time-Resolved Photoluminescence Spectroscopy τ ≈ 1/e  Part III: Spectroscopic Characterization Pump Probe Spectroscopy ✓ Some processes occur on time scales as short as a few picoseconds (10-12 s) or femtoseconds (10-15 s). Observation of these fast processes is essential to fully understanding the dynamics of various excitations in matter. ✓ To accurately measure ultrafast processes, the uncertainty in timing must be smaller than the time scale of the process, requiring temporal resolution on the order of 10-15 s ✓ Thus to probe processes in the femtosecond timescale we can use pump-probe spectroscopy with a femtosecond pulsed laser  Part III: Spectroscopic Characterization Pump Probe Spectroscopy ✓ Pump-probe spectroscopy, involves observing the state of a process indirectly through observation of a “probe” laser pulse ✓ The first step in performing pump-probe measurements is to use a beam splitter to split a laser pulse into a ‘pump’ pulse and a ‘probe’ pulse ✓ Each of the two pulses approach the sample on different paths determined by the experimenter through placement of mirrors  Part III: Spectroscopic Characterization Pump Probe Spectroscopy Possible types of signals obtained in a pump-probe transient absorption experiment. The absorbance (or optical density) of the excited sample minus that of the unexcited is plotted as a function of probe wavelength. Depopulation of the ground state (S0) leads to a decreased absorbance (or increased transparency), which is due to the ground state bleach (GSB). Excited state absorption (ESA) gives rise to an increased absorbance, whereas stimulated emission (SE) gives rise to a decreased absorbance, just as does GSB.  Part III: Spectroscopic Characterization Pump Probe Spectroscopy  Part III: Spectroscopic Characterization Pump Probe Spectroscopy Part IV: Electrical characterization of nanomaterials films Part IV: Electrical Characterization of nanomaterials films Electrical characterization measurements are very rigorous: very sensitive to any defects, contaminants in the film. These defects will cause scattering effects during electron transport and decrease mobility Electrical characterization can reveal some hidden traps that cannot be seen by morphological measurements Part IV: Electrical Characterization of nanomaterials films Sheet resistance, is the resistance of a square piece of a thin material with contacts made to two opposite sides of the square. It is usually a measurement of electrical resistance of thin films that are uniform in thickness. It can be used to characterize nanomaterial-bases films The sheet resistance characterises electrically a thin film (ideal for graphene or graphene based materials): 𝐿 𝐿 ρL L 𝑅=𝜌 =𝜌 = = R sh (A is the cross section) 𝐴 𝑊𝑡 t W W 𝜌 𝑅𝑠ℎ = in Ω/sq 𝑡 The sheet resistance of films can be measured using the 4-probe technique Part IV: Electrical Characterization of nanomaterials films 4-point probe Current is applied across the two outer electrodes (1 & 4), whereas the potential drop across the two internal electrodes is measured (2 &3) In the case (a) the film is thinner than the 40% of the electrodes distance (S); and (b) the lateral size of the sample is large, the RSH equals to: π ΔV 𝛥𝑉 𝑅𝑆𝐻 = = 4.53236 ln2 I 𝐼 Part IV: Electrical Characterization of nanomaterials films 4-point probe Part IV: Electrical Characterization of nanomaterials films 4-point probe Part IV: Electrical Characterization of nanomaterials films 4-point probe Summary Part I: Surface Characterization Optical Microscopy (OM) Scanning Electron Microscopy (SEM) Scanning Tunneling Microscopy (STM) Atomic Force Microscopy (AFM) Profilometry Part II: Structural Characterization TEM (Transmission Electron Microscopy) X-ray Diffraction Part III: Spectroscopic Characterization Fourier Transform Infrared (FTIR) Spectroscopy Raman Spectroscopy Ultraviolet-Visible (UV – VIS) Spectroscopy X-ray photoelectron spectroscopy (XPS) Photoluminescence (PL) Time-resolved Photoluminescence (TRPL) Pump-probe spectroscopy (PPS) Part IV: Electrical Characterization of nanomaterials films 4-point probe