Download LAB UNIT 5: Scanning Tunneling Microscopy and more Summaries Chemistry in PDF only on Docsity!
LAB UNIT 5: Scanning Tunneling Microscopy
Specific Assignment: STM study of HOPG and Gold films
Objective This lab unit introduces scanning tunneling microscopy (STM) technique, used to obtain real space atomic resolution images of conductive surfaces. The tunneling spectroscopy mode of STM is employed to examine local density of state (LDOS) of the surface.
Outcome Learn about the basic principles of scanning tunneling microscopy, including a short introduction of the tunneling phenomena, and learn how the STM images can be correctly interpreted. Attain STM images and the local density of state of a HOPG (highly ordered pyrolytic graphite) and gold (Au) sample in ambient atmosphere.
Synopsis The STM provides real space atomic resolution images through tunneling current between a conductive tip and a conductive/semiconductive surface. In this lab unit, we employ two STM modes, i.e., constant current imaging mode and tunneling spectroscopy mode, to study HOPG (graphite) and gold (Au). HOPG is one of well studied materials and serves as a standard for STM technique, and the interpretation of the STM images as well as the spectroscopic analysis are debated actively in literatures. Here, taking into consideration of artifacts such as thermal drift, students will determine the lattice constant and the atom-to-atom distances of HOPG. The contrasting spectroscopic data of the HOPG and Au will illustrate the difference in electronic structure between semi-metals and metallic systems.
A STM image (Left, 9 Å x 9 Å ) and a voltage dependent tunneling spectroscopy curve (Right) of HOPG
Materials Highly Ordered Pyrolytic Graphite (HOPG) and Gold (Au) film
Technique STM in imaging mode and tunneling spectroscopy mode
Table of Contents
- Assignment.................................................................................................
- Quiz – Preparation for the Experiment
- Theoretical Questions..............................................................................................
- Prelab Quiz
- Experimental Assignment
- Goal
- Safety.......................................................................................................................
- Instrumental Setup...................................................................................................
- Materials..................................................................................................................
- Experimental Procedure
- Background: Local Electronic Properties and STM................................
- Motivation
- Scanning Tunneling Microscopy
- Tunneling Spectroscopy..........................................................................................
- Layered Structure of HOPG....................................................................................
- References
- Recommended Reading...........................................................................................
2. Quiz – Preparation for the Experiment
Theoretical Questions
(1) Sketch the tunneling phenomena between a metallic STM tip and a metallic sample surface at (a) no bias voltage, (b) positive voltage, and (c) negative voltage. (2) How does a contamination of a STM tip, with organic molecules for example, influence the tunneling current, i.e., the tunneling barrier? Discuss. (3) Sketch the electronic structures and I-V curves of tunneling spectroscopy of the four systems; metallic, semi-metallic, semiconductive, and non-conductive. (4) What is “three-fold-hexagon” of HOPG? Explain.
Prelab Quiz
(1) (6pt) The STM image (below) of a HOPG shows honeycomb structure, known as “three-fold-hexagon” pattern. Determine the lattice constant and the atom-to-atom distance of HOPG of the STM image below.
(1.4 nm x 1.4 nm)
(2) (2pt) An actual I-V curve of a HOPG sample is shown below. Sketch the differential conductance (dI/dV)/(I/V) of this I-V curve in the given space.
(3) (2pt) List the reasons why the atomic structure of gold sample is difficult (or impossible for our lab) to obtain?
3. Experimental Assignment
Goal
Following the step-by-step instruction below, obtain the STM images and determine the characteristic lattice constant of HOPG. Analyze the tunneling spectroscopy data to determine the conductivity of the systems. Analyze and discuss the data with the background information provided in Section 4. Provide a written report of this experiment.
Specifically provide answers to the following questions: (1) According to the analysis, what were the lattice constant and the atom-to-atom distance of the HOPG? (2) Compare the values obtained in (1) with the literature values. How closely does your result agree/disagree that of the literature values? Discuss your findings. (3) Show the STM images that were obtained at different bias voltage. Discuss how and why they are different/ indistinguishable. (4) According to the spectroscopic analysis, what type of system is HOPG? How about Au? Explain your conclusion. (5) STM has been applied to image DNA and other biological macromolecules, which are in general not conductive. How would you image a single biological molecule place on gold substrate?
Safety
- Wear safety glasses.
- Refer to the General rules in the AFM lab.
- Wear gloves when handling ethanol.
Instrumental Setup
- Easy Scan 2 STM system with 0.25nm (diameter) Pt/Ir wire (STM tip)
- STM granite vibration isolation platform
Materials
- Samples: Highly Ordered Pyrolytic Graphite (HOPG) and a gold film. Samples are kept in designated containers when they are not used to avoid contamination.
- Ethanol in squeeze bottle and cotton swabs for cleaning.
- Scotch tape for cleaving the HOPG layers.
Experimental Procedure
Read carefully the instructions below and follow them closely. They will provide you with information about (i) preparation of the experiment, (ii) the procedure for attaining the STM images, (iii) attaining the tunneling spectroscopy data, (iv) the procedure for closing the experiment, and (v) on how to process/analyze the STM images and to process spectroscopy data.
Figure 3.2: Installing the tip into the STM head.
(4) Install the sample. a. Remove the sample holder from the storage container by holding the black plastic part. DO NOT TOUCH the metal part. b. Check for any contamination (dust, fingerprint) on the metal part. If cleaning is necessary, follow the cleaning procedure. i. Moisten a cotton swab with ethanol and gently clean the surface. ii. Allow the alcohol to completely dry. c. Place it on the sample holder guide bar of the STM head. Make sure it does not touch the tip. d. Cleave the HOPG (graphite) sample. (Figure 3.3) i. Stick a piece of scotch tape gently to the graphite and then gently press with the back, flat part of the tweezers. ii. Pull the tape off. The topmost layer of the sample should stick to the tape, leaving a freshly exposed graphite surface. iii. Remove any loose flakes with the part of tweezers.
Figure 3.3: Cleaving the graphite sample.
e. Using a tweezers, hold the graphite sample at the magnetic pak. f. Take the sample holder (handle at the black plastic part), and place the graphite sample on the magnet. g. Place the sample holder back on the STM head. Make sure it does not touch the tip.
Figure 3.4: Placing the sample on the sample holder.
(5) Turn on the Controller main power switch. (6) Open the Easy Scan 2 control software. (7) In the operation mode panel, select STM.
(ii) Procedure for attaining the STM images (1) Coming in contact. a. Push the sample holder carefully to within 1mm of the tip. The tip should not touch the sample. b. Look into the graphite surface. There should be a small gap between the very end of the tip and the reflection of the end of the tip.
Figure 3.5: Coarse approach.
c. Open the Positioning window. d. Through the magnifier, watch the distance between the tip and sample as click Advance in the approach panel. The tip should be within a fraction of a millimeter to the surface (i.e., the reflection of the tip). e. Set control parameters in Z-control panel: Set point 1nA, P-gain 10000, I- gain 1000, Tip voltage 50 mV. f. Click Approach.
(5) Take the sample off of the sample holder. Place the sample in its case. (6) Clean the sample holder with ethanol and a cotton swab. Let it dry. (7) Place the sample holder in the case. Close the cap tightly. (8) Place the STM cover over the STM head. (9) Turn off the controller power switch.
(v) Instruction for data analysis
(1) Open the Easy Scan 2 program to process images. Save the images with scale bar. (2) The Report program is also used to measure the lattice distance and the atom-to- atom distance of HOPG sample. It will be helpful to also to show the images with the measuring lines, etc. in your final report. So save images as you take measurements (to be imported to your report). (3) Open the spectroscopy files with Excel. (4) Generate columns: ln( I ), ln( V ), and d(ln( I ))/d(ln( V )). Calculate ln | I | and ln | V |. (5) Create plot of ln( I ) vs ln( V ). (6) Using Add trendline function, obtain the fit curve for the ln( I ) vs. ln( V ) curve .Use either a 2nd^ order or 3rd^ order polynomial, whichever gives a better fit. (7) Differentiate the fit curve that is equal to d(ln( I ))/d(ln( V )). Type the derivative equation in the cells of the d(ln( I ))/d(ln( V )) column. (8) Create a plot of I vs. V (current as a function of voltage, raw data), and differential conductance ((d I /d V )/( I / V ) = d(ln( I ))/d(ln( V )) as a function of voltage. (9) Also Plot I - z curve, i.e. current as a function of the z distance in semi-log scale.
(10) With Equation 2, log( I )=− A φ⋅ z + C , determine the barrier height φ.
4. Background: Local Electronic Properties and STM
Table of Contents Motivation ......................................................................................................................................... 127 Scanning Tunneling Microscopy....................................................................................................... 127 Tunneling Spectroscopy .................................................................................................................... 131 Layered Structure of HOPG .............................................................................................................. 133 References ......................................................................................................................................... 134 Recommended Reading..................................................................................................................... 134
Motivation
With the development of quantum mechanics in the early 20th^ century, mankind’s perception of nature was stretched to a great degree leading to new axioms, and the recognition of the particle-wave dualism. It was found that particles with small masses such electrons could interchangeably be described as waves or as corpuscular objects. With the wave character of matter, particles exhibit a probability of existence at places, where they can classically not exist. One of these phenomena is the tunnel effect, which describes the ability of an electron to tunnel through a vacuum barrier from one electrode to the other. Since 1960 tunneling has been extensively studied experimentally. This led in 1981 to the first microscopic tool with which atoms could be observed in real space – the scanning tunneling microscopy. In addition to the atomic resolution imaging capability of STM, tunnel currents could be studied with this tool in a spectroscopy manner providing insight into the local density of state (LDOS) of material surfaces.
Scanning Tunneling Microscopy
While vacuum tunneling was theoretically predicted by Folwer and Nordheim 1928,^1 , it was not until 1981 with G Binnig and H. Rohrer’s introduction of the scanning tunneling microscope (STM) that provided the first observation of vacuum tunneling between a sharp tip and a platinum surface.
Wavefunction Overlap, Electron Probability STM is based on a quantum mechanical phenomenon, called tunneling. In quantum mechanics, small particles like electrons exhibit wave-like properties, allowing them to “penetrate” potential barriers, a quantum mechanical probability process that is based on classical Newtonian mechanics impossible.*^ In general, STM involves a very sharp conductive tip that is brought within tunneling distance (sub-nanometer) of a conductive sample surface, thereby creating a metal-insulator-metal (MIM) configuration. In the representation of one-dimensional tunneling (Figure 4.1), the
tunneling wave of the sample electrons, ψ s, and the wave of a STM tip electrons, ψ t,
overlap in the insulating gap, allowing a current to flow.
To achieve some understanding of the physical meaning of the wave function ψ,
we consider the square magnitude of it, which represents the probability of finding an electron at a given location. Generally, this is visualized with electron clouds for atoms or
- (^) A more detailed discussion on barrier tunneling is provided in a later section of this text.
( )exp( 1. 025 )
( )exp 2 V E z
m E z I Vbias s EF ∝ bias s F − ⋅ ⎥
where m is the mass of electron and ħ is the Planck’s constant. An electronic state describes a specific configuration, an electron can possess. For instance, it can have either a spin up or spin down, or a particular magnetic momentum etc. A state is described by a set of quantum mechanical numbers. Each state can only be filled by one electron. Consider a classroom of X chairs with Y < X students. The chairs represent the states and the students the electrons. Let us assume, it is hard to read the board, and the students are all very interested in the subject. Consequently the chairs will be filled up towards the front with some empty seats in the back. This situation is illustrated in Figure 4.3. The chairs in each row are represented by circles. Filled circles represent student occupied chairs. The distance from the board is indicated with x. The number of chairs per row represents the density of states (DOS) for a particular classroom. Two distinctly different classrooms are shown in Figure 4.3. In the second classroom N is a function of the x. The last row that is filled is identified by x (^) F. Returning
to electrons in metals; x F corresponds to the Fermi energy Ef , N to ρ s and N(x F) to ρ s ( Ef ).
In the case of the free electron model for s-/p-metals at zero Kelvin, ρ s ( E ) is proportional
to the square root of the energy.
N = 5 5 5 5 5 5 N = 6 6 5 3 3 2 1
Figure 4.3: Density of state (DOS), N, and Fermi energy x (^) F in two classroom settings. (left) N is constant. (right) N(x).
Many physical properties are affected or depend on the number of states within an energy range (i.e., the energy density of states). While in metals and semi-metals, there is relatively small variation in the density of states due to the large electron delocalization, the density of energy levels in semiconductors varies noticeably. Thus, knowledge about DOS is of immense importance for electronic applications involving semiconducting materials, where the availability of empty valence and conduction states (states below and above the Fermi level) is crucial for the transition rates. In comparison to Figure 2 that visualizes tunneling between metals, Figure 4.4 illustrates the tunneling mechanism involving a semiconductor. The filled area (grey) is not uniform, representing the variation in electron density, and the lines in the unoccupied levels represent the variation in density of the energy levels that the tunneling electrons can occupy.
x x (^) F
x x (^) F
(a) (b) Figure 4.4: (a) Schematic of a metal-insulator-semiconductor tunneling junction and (b) corresponding normalized differential tunneling conductance.
STM and Local Density of States STM constant current maps provide information about the variations in the electron density, and do not necessarily correspond to the location of atoms (nuclei). Figure 4.5 illustrates that a location of high tunneling current in a STM image can be either a compounded affected of two atoms, leading to a current maximum in between the atoms, or be identical with the location of an atom. This is for instance found for the silicon (001) 2x1 surface.^3 A π molecular orbital of the silicon-silicon dimers (Si=Si) creates the highest electron density (probability) at the center of the dimers, while an anti- bonding π* molecular orbital has a node (a location where the probability is zero) at the center of the dimers. Thus, when a negative bias is applied, the electrons in the π- molecular orbital (occupied state) tunnel and the resulting image, similar to the case shown in Figure 4.5(a), will be obtained. When a positive bias is applied, the electrons of the tip tunnel into the anti-bonding π* molecular orbital (unoccupied state), revealing a gap between the dimers, as in Figure 4.5(b). When the variation in the local DOS (LDOS) of metals is small, the contour of STM images often can be safely interpreted as the topography of the atomic lattice. †
(a) (b)
Figure 4.5: Sketch of possible STM images relative to the nucleus locations. Top view is the contouring lines of STM images and the corresponding side view on the bottom. STM image shows high tunneling location (a) at center of two nuclei and (b) at the top of each nucleus.
† (^) See next section on LDOS on variety of systems.
spectroscopy on silicon 111 (7x7) surface is location specific. 4 Interestingly the average I-V curves at various locations closely resembles to data obtained by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). It suggests that UPS and IPS are the area average of the differential conductance, while STM tunneling spectroscopy is capable of resolving local information, e.g. local DOS rather than average DOS. The general profile of the density of state around the Fermi level, i.e., (d I /d V )/( I / V ), can be used to classify the material based on its conductivity, as illustrated in Figure 4.7. As shown, metals do not possess a gap between the occupied states (valence band) and the unoccupied states (conduction band) and the variation in DOS is relatively small. Thus, the I-V curves are linear for the most part, resulting in a very small dI/dV gradient. Semi-metals also do not have a gap between the occupied and unoccupied states. There is, however, a gap in the momentum space (the waves are out of phase) that depresses the conductance around the Fermi level, and consequently bends the density of states at low voltages. For semiconductors and insulators, the conductance around the Fermi level is zero. The threshold voltage, i.e., band gap, Eg = |V+bias | + |V-bias |, is relatively small for semiconductor (< 3eV, used as definition for semiconductors). As shown in Figure 4.7, semiconductors show a highly bend DOS, which is flat as for insulators at low voltages, where the energy gap Eg cannot be bridged. It is well known that doping semiconductors with impurities or defect sites affect reduce Eg , and thus, can modify the density of states at the Fermi level to such a degree that it resembles nearly a semi-metal.
Figure 4.7: The electronic structures and corresponding IV curves and dI/dV curves of tunneling spectroscopy.
So far, we have discussed elastic tunneling spectroscopy, in which the energy of the tunneling electrons is conserved. In inelastic tunneling spectroscopy, the counter electrode is not the material under investigation; rather it is the gap that is examined. In general, the material of interest is placed on top of the counter electrode or fills the insulating gap completely as a thin film. When the tunneling current travels through the material, a part of tunneling electron energy is dissipated by activating various modes of the molecular motion, e.g. C-H stretching of hydrocarbon chains. Thus the modes of the molecular motion can be deduced based on the extensive data base of infrared spectroscopy (IR). Experimentally, the I-V curve is obtained in the same manner as the elastic tunneling spectroscopy. To identify the modes of the molecular motion, the second derivative, dI^2 /d^2 V , is calculated, which contain multiple number of sharp peaks. The modes of molecular motion are then identified by the locations of the peak Vpeak.
Layered Structure of HOPG
Highly ordered pyrolytic graphite (HOPG) consists of layers of carbon sheets, forming a semi-metallic system. While the carbons within a sheet are covalently bonded to form a hexagonal lattice structure, the layers are held together by Van der Walls forces. The in-plane lattice constant (repeating unit length) and the z-axis lattice constant are 2.46Å and 6.7 Å respectively and the in-plane atom-to-atom distance is 1.42 Å. The sheets are arranged such that the every other carbon on a layer has a carbon in the neighboring sheets, Figure 4.8. The carbons in the first layer that have a carbon in the second layer right below are called an A-site carbons, and the carbons without a carbon directly below are called B-site carbons.
Figure 4.8: Layered structure of HOPG.
In STM images, the two types of carbons (A-site and B-site) appear differently. As shown in Figure 4.9, the B-site carbons exhibit a higher LDOS (i.e., topography) than the carbons at the A-site, exhibiting the three-fold-hexagon pattern.
