Kevin F. Kelly. A Thesis ... to observe electron scattering from point defects and steps on graphite. This cannot ..... The crystal structure is face-centered cubic with a lattice constant of ...... crystallographic directions along the graphite surface.
RICE UNIVERSITY
Functionalization of Scanning Tunneling Microscope Probes with Buckminsterfullerenes by
Kevin F. Kelly A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree
Master of Science Approved, Thesis Committee:
Naomi J. Halas, Director Associate Professor of Electrical Engineering Peter Nordlander Associate Professor of Physics Richard E. Smalley Professor of Chemistry and Physics Houston, Texas May, 1996
Functionalization of Scanning Tunneling Microscope Probes with Buckminsterfullerenes Kevin F. Kelly
Abstract This dissertation analyzes the feasibility and advantages of molecular functionalization of scanning tunneling microscope (STM) probes with buckminsterfullerenes. The C60 molecules are adsorbed onto the tunneling region of a STM tip by rastering the tip in a thin lm of C60 vacuum deposited on graphite. The individual tip-adsorbed molecules are subsequently imaged in situ by scanning the fullerene-adsorbed tip over a defect covered graphite surface. These nanometer-size defects serve as a surface tip array which images the molecules adsorbed to the tip when the surface is scanned. It is then demonstrated that the fullerene-adsorbed STM tips can be used to observe electron scattering from point defects and steps on graphite. This cannot be observed using bare metal tips. Molecular functionalization adds a new dimension to scanning tunneling microscopy by allowing greater control of the electronic interactions between the tip and sample.
Acknowledgments In graduate school, as in the elds of science and engineering, one only succeeds by standing on the shoulders of his predecessors and peers. I would like to gratefully acknowledge the many shoulders that I stood upon, cried upon, and leaned against during the course of my research. First and foremost is my research advisor and mentor Naomi Halas. Without her constant guidance and support this research would never have occurred. Next, I would like to thank Peter Nordlander for his theoretical insights into tunneling theory and speci cally C60 nanotips. And I would also like to acknowledge the third member of my graduate committee, Richard Smalley, whose discovery and knowledge of C60 was essential to this research. As important as my research committee, are my fellow graduate students, whose presence was as valuable and appreciated outside the lab as much as it was within. Jay Resh, who is the father of the STM research in the lab of Dr. Halas. Dip Sarkar, the consummate engineer and scientist who designed the STM system that I built and used during my research. Rick Averitt and Phil Pippenger whose assistance as senior graduate students was only surpassed by their assistance as weight-lifting trainers. Greg Hale, my peer and spiritual guide. Steve Oldenburg, a great companion during working hours and happy hours.
iv I would also like to thank a few researchers from other universities and research labs that I met and had a profound in uence on my research. Dr. Ohno, at my undergraduate institution, rst introduced me to the pains and pleasures of scanning tunneling microscopy and for that I am forever in his debt. Dr. Rohrer, his visit to Houston and enthusiasm for this research was inspirational. A special note of thanks goes to Dr. Mizes for many useful discussions, and without whose theory, this thesis would be just another collection of anomalous STM images. I would also like to acknowledge Dr. Avouris and Dr. Eigler, who, during their visits to Rice, took the time to discuss with me many aspects of their research and mine. Most importantly, I would like to thank my family for their love and support during my research and throughout my life.
v
Contents Abstract Acknowledgments List of Illustrations
1 Introduction
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2.1 Introduction : : : : : : : : : : : : : 2.2 Theory of STM : : : : : : : : : : : 2.2.1 General Theory : : : : : : : 2.2.2 STM Imaging of Graphite : 2.3 Preparation of Fullerene STM Tips
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1.1 1.2 1.3 1.4
Introduction : : : : : : : : : : : Scanning Tunneling Microscopy C60 Properties : : : : : : : : : : Outline : : : : : : : : : : : : : :
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2 Experimental
3 Inverse Scanning Probe Microscopy 3.1 Introduction : : : : : : : : : : : : 3.2 Inverse AFM : : : : : : : : : : : 3.3 Inverse STM : : : : : : : : : : : : 3.3.1 Defects on Graphite : : : 3.3.2 Inverse STM of fullerenes
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4 Electron Scattering on Graphite
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4.1 Introduction : : : : : : : : : : : : : : : : : 4.2 Theory of Electron Scattering on Graphite 4.3 Imaging with C60 Tips : : : : : : : : : : : 4.3.1 Experimental Results : : : : : : : : 4.3.2 Imaging Mechanism : : : : : : : : :
5 Future Directions
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5.1 Introduction : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 5.2 Inverse Imaging : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 5.3 Electron Scattering on Graphite : : : : : : : : : : : : : : : : : : : : :
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vi 5.4 Fullerene STM Tips : : : : : : : : : : : : : : : : : : : : : : : : : : : :
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Bibliography
49
vii
Illustrations 2.1 Real part of the electron wavefuntion in 1-D tunneling : : : : : : : : 2.2 Crystal structure of hexagonal graphite : : : : : : : : : : : : : : : : : 90 2.3 A 90 A A STM image of graphite obtained with a Pt/Rh tip. The tunneling parameters were 3 nA tunneling current and -100 mV sample bias. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 2.4 A 75 A 75 A STM image of corrugation reversal on graphite. 2 nA, -100 mV : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10
A 45 A 45 A STM image of a point defect on graphite. 2 nA, -100 mV A 90 A 90 A STM image of a superlattice defect. 1 nA, +200 mV A 60 A 60 A STM image of a subimplantation defect. 4 nA, -100 mV A 77 A 77 A STM image of an amorphous defect. 2 nA, +100 mV A 600 A 600 A Inverse images of a bare Pt/Rh STM tip. 1 nA, +100 mV : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : A 220 A 220 A scan with inverse images of a fullerene-adsorbed STM tip. 1 nA, +100 mV : : : : : : : : : : : : : : : : : : : : : : : : A 200 A 200 A scan of a three fullerene tip. 1 nA, +100 mV : : : : Inverse images of the same tip approximately one minute later. Same scan parameters : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : A 65 A 65 A image of two fullerenes on adsorbed on a tip. 1 nA, -100 mV : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : A 80 A 80 A scan of a single fullerene on the tip. 1 nA, +100 mV :
4.1 Electron scattering on graphite observed with a C60 tip. 60 A 60 A ,1 nA, -100 mV : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 4.2 Electron scattering on graphite observed with a fullerene-adsorbed STM tip. 85 A 85 A ,1 nA, +100 mV : : : : : : : : : : : : : : : : 4.3 Theoretical image of electron scattering from Mizes and Foster.[14] : 4.4 Theoretical image of electron scattering from Mizes and Foster.[14] : 4.5 Depedence of threefold scattering on tip bias voltage. : : : : : : : : : 4.6 A 100 A 100 A image of electron scattering from a graphite step. 2 nA, +100 mV : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :
9 14 15 16 24 24 25 25 26 28 29 29 30 31 37 37 38 38 39 40
1
Chapter 1 Introduction 1.1 Introduction Since its invention, the scanning tunneling microscope (STM) has become a powerful tool in many elds of science and engineering. It was the rst in a series of scanning probe microscopies to investigate the surface properties of materials on a nanometer scale. The STM can consistently achieve atomic resolution on a wide range of conducting and semi-conducting materials. In recent years, it has evolved from a local probe to also become a tool in nanometer lithography[1] and manipulation.[2, 3] The atomic resolution of the STM makes it a valuable surface probe for physicists, but its ability to work in air and water has also made it a useful tool for chemists and biologists. However, even now, 14 years after its invention, the understanding of the physics at the STM junction is incomplete. One step toward a better understanding of the tunneling mechanism is to gain more control of the probe. With a known electronic structure for the probe, much better interpretation of the STM images can be made.
1.2 Scanning Tunneling Microscopy Scanning tunneling microscopy is based on the quantum mechanical phenomenon of tunneling. Tunneling, or barrier penetration, occurs when a particle which classically
2 does not have enough energy to traverse a barrier can still penetrate the barrier and continue on the other side. In STM, the tunneling particles are electrons and the barrier is the gap between the probe apex and the sample surface. Classically, an electron current could only ow between the tip and the sample if they were in contact. Due to the quantum mechanical wave nature of electrons, however, the electrons can tunnel through the gap before contact, allowing the STM to investigate the surface nondestructively. The probability of nding the particle on the other side of the barrier decays exponentially with the barrier distance. It is this exponential dependence of the tunneling phenomenon that accounts for the subnanometer lateral and vertical resolution of the STM. The most common probe in STM is a sharpened transition metal wire, usually composed of tungsten, gold, or a platinum alloy. This probe is brought to within a nanometer of the sample surface by a piezoelectric scanner. Piezoelectric ceramics have the abililty to lengthen and contract with distances less than anangstrom when a voltage is applied. Subnanometer control (0.1 Avertical, 1 Alateral) of the STM tip is necessary for maintaining the tunneling gap and scanning the surface. By applying a bias voltage across the junction of the tip and sample, a current composed of the tunneling electrons will ow. The image generated from rastering the tip over the surface depends upon both the occupied states where the electrons originate and the unoccupied states into which they are collected. Because the image is a combination of both geometric and electronic structure, the interpretation of STM images is not always straightforward. In addition, because of the exponentially decaying distance dependence of the tunneling current, the image generated by the STM is very sensitive
3 to the properties of both the tip and sample. While sample preparation methods have been well established in the eld of surface science for other experimental techniques, the preparation of the STM probe is not as developed. One of the current goals in the eld of scanning probe microscopies is to gain a better understanding and more control of the probe itself. Generally, the emphasis has been on etching and sharpening procedures to obtain more reliable probes. While this is a good procedure for observing larger features, it is still dicult to gain insight into the tip e ects during atomic scale imaging. The reason for this is a lack of knowledge of the metal probe's topographic and electronic properties at the tunneling site. Another option is to functionalize a tip with molecules, where the molecules govern the tip-sample interaction and the rest of the tip acts only as a supporting framework. Molecular functionlization of probes has the advantage that the in uence of the probe on the surface can be customized by the choice of di erent molecules. However, the molecular functionaization technique has been limited to atomic force microscopy (AFM). The research in this thesis describes the preparation, characterization, and utilization of tips functionalized with C60 molecules for use in STM.
1.3 C60 Properties C60 , or buckminsterfullerene, consists of 60 carbon atoms at the vertices of a regular truncated icosahedron. The average C-C bond length in this \soccer ball" geometry . The sixty valence electrons in orbitals of s0 1 p character are orthogonal is 1.44 A :
to the surface and give C60 a closed electron shell [4]. The C60 molecule, including
4 the delocalized -electrons, has a diameter of 10.18 A. In the isolated molecule, the HOMO-LUMO gap is approximately 1.9 eV. In the solid state at room temperature, the C60 molecules are bound by van der Waals forces. The crystal structure is face-centered cubic with a lattice constant of 14.15 A[5]. The molecules freely rotate about arbitrary axes of rotation at room temperature, thus giving solid C60 a degree of rotational disorder. However, solid C60 undergoes a phase transition at 260 K to a simple cubic lattice, with a lattice constant .[5] In the 90 - 260 K temperature range, the molecules jump between two of 14.10 A energetically favorable con gurations. Below 90 K the molecules lock into a simple cubic lattice. Energy band calculations indicate a direct gap of 1.5 eV for solid C60 .[6] There are many reasons for the choice of C60 molecules in the functionalization of STM tips. First, the rotational motion of C60 is frozen by its charge transfer bonding to metal surfaces. This allows the fullerenes to be adsorbed directly on the tip without
a priori preparation. Tips prepared in this way have shown atomic resolution on graphite;[7] therefore, there is no sacri ce of resolution with fullerene-adsorbed STM tips. The spatial extent of the fullerene allows the tunneling junction between it and the sample to be located far from the bare metal tip. Therefore, contributions from the tunneling states of the metal would be negligible due to the exponential dependence of the tunneling current. The many studies on the covalent functionalization of C60 allow for a large variety in the choice of tip electronic properties.[8] Langmuir-Blodgett and self-assembly techniques of fullerenes[9, 10, 11] also allow for the possibility of immobilizing the fullerenes on the STM tip in uniform monolayer lms, similar to the molecular modi cation that has been done on AFM tips.[12] Lastly, C60 molecules
5 can be elastically compressed between the STM tip and sample and still return to their original structure when the pressure is released.[13] Structural stability of the molecular tips is necessary due to the \tip crashes" that can occur during scanning.
1.4 Outline As mentioned above, previous research in this group has demonstrated that attaching fullerene molecules to the apex of a STM tip results in improved atomic resolution on graphite as compared to bare metals tips.[7] However, the exact mechanism of the improved resolution was not determined due to the diculties in analyzing the interactions at the STM junction. In this thesis, the changes induced by the adsorption of C60 onto a STM probe is further investigated. Chapter 2 will begin with a review of tunneling theory as it applies to the STM and will also discuss the STM used in this study. Futhermore, it will cover the creation of the fullerene coated tips that are the focus of this thesis. To con rm that the improved resolution is due to tunneling through the fullerene and not to mechanical e ects, the fullerene STM tips are inverse imaged during the tunnneling process by a surface tip array. This inverse imaging technique and its results will be discussed in Chapter 3. Chapter 4 describes further e ects of tunneling through a fullerene, with images of electron scattering from point defects on graphite. Predicted in 1989 by Mizes and Foster,[14] these images have not been previously seen experimentally and cannot be imaged with a bare metal STM tip. Besides point defects, images of electron scattering from steps on graphite have also been viewed with fullerene-adsorbed STM tips. The theoretical mechanisms responsible for the imaging of electron scattering with a fullerene tip will also be dis-
6 cussed. Chapter 5 will summarize the present research, and the future directions and possibilities of this work.
7
Chapter 2 Experimental 2.1 Introduction This chapter will be an overview of the various components of the experimental apparatus used for this research. The most important of these is the scanning tunneling microscope. Since this thesis describes a new utilization of the scanning tunneling micrscope, a brief but thorough theoretical review of the STM is necessary. This will also include the current theory of imaging graphite with the STM. The second section of this chapter discusses the preparation of the fullerene-adsorbed STM tips that are the focus of this thesis.
2.2 Theory of STM 2.2.1 General Theory The scanning tunneling microscope uses electrons instead of photons to produce images and probe di erent properties of surfaces. But unlike other electron microscopies, it utilizes quantum mechanical tunneling to achieve its image resolution. This phenomenon is covered in many introductory quantum mechanics and scanning tunneling microscopy texts. [15, 16] For simplicity, we will begin with a free particle traversing a one-dimensional barrier. This e ect will then be explained in the context of the
8 tunneling of electrons in solids. After which, a summary of the current theoretical descriptions of the STM will be presented. To describe tunneling through a one dimensional barrier, we begin with the Schrodinger equation for a free particle, in this case an electron,
; 2hm dd2x (x) + U (x) (x) = E (x); 2
2
where m is the electron mass, 0.511
M eV c2
(2:1)
. We then introduce a potential barrier
located at the origin that has a height U and a width a, and is zero everywhere else. If an electron with energy E < U is incident from the left with these initial conditions, Eq. 2.1 can be solved to nd the wavefunctions of the electron to the left, to the right, and inside the potential barrier. Outside the barrier, the wavefunction of the electron has the form (x) / exp (ikx);
(2:2)
where
p
k = 2hmE (2:3) is the wave vector. This solution describes the electron as traveling with a constant velocity. Classically, the electron is completely re ected at the barrier, however, this is not the case in quantum mechanics. Inside the potential barrier, where the particle is classically forbidden, Eq. 2.1 has a solution (x) / exp (;x);
(2:4)
9 where q 2m (U ; E ) = h
(2:5)
is the decay constant. This describes the state of an electron decaying exponentially in the +x direction. Thus the electron has a nonzero probability of penetrating and traversing the barrier as shown in Fig 2.1. We can extend this simple model to the case of metal-insulator-metal tunneling to obtain a few qualitative features. Assuming the di erence between the electron's energy and the barrier potential is approximately the work function of a typical metal (4 eV) and assuming a bias voltage much less than the work function, takes on
Ψe
a
Figure 2.1: Real part of the electron wavefuntion in 1-D tunneling
10 a value around 1 A;1 . The tunneling current is then proportional to the probability denisty which is itself the square of the wavefunction,
j (x) / exp ;2x:
(2:6)
This tunneling current is very sensitive to changes in the barrier width, varying approximately one order of magnitude in current for every 1 A change in the barrier width. It is this exponential dependence of the tunneling current that gives the scanning tunneling microscope its high vertical and lateral resolution. A more theoretical calculation of tunneling between two planar metal electrodes was developed by Bardeen applying a time-dependent perturbation approach that uses a many particle formalism.[17] Bardeen rst considers the two electrodes separately and solves the stationary Schrodinger equations for each subsystem. The tunneling current is then calculated through the overlap of the wavefunctions of the free systems using Fermi's Golden Rule. With a bias voltage V , the total tunneling current is
I = 4he
Z1 ;1
[f (E ; eV + ) ; f (E + )] (E ; eV + ) (E + )j M j2d; f
f
s
f
t
f
(2:7)
where f (E ) = f1 + exp [(E ; E ) =k T ]g;1 is the Fermi distribution function, (E ) f
b
s
and (E ) are the density of states (DOS) of the two electrodes, and M is the tunneling t
matrix element between the unperturbed electronic states of the two electrodes. The essential problem is the calculation of the tunneling matrix element. Bardeen showed
11 that M is determined by a surface integral on a separation surface between the two electrodes, z = z , o
!
Z
M = @@z ; @ (2:8) @z dS; where and are the wavefunctions of the two electrodes. For the case when k T is b
small, the Fermi distribution function can be approximated by a step. Eq. 2.7 then becomes and integral between 0 and eV . Bardeen further assumed that the magnitude of the tunneling matrix element is constant over this interval. The tunneling current is then proportional to the convolution of the DOS of the two electrodes.
I/
Z
eV
0
(E ; eV + ) (E + )d: s
f
t
(2:9)
f
Building upon Bardeen's theory of tunneling between electrodes, Terso and Hamann devoloped a three-dimensional treatment of the tunneling between a STM tip and sample. [18] The tip was modeled as a spherical potential well and the tunneling matrix element was evaluated for a s-type tip wavefunction. The limits of low temperature and small applied bias voltage were also assumed. With these assumptions, it was shown that a constant current STM image corresponds to a map of the constant local state density of states (LDOS) at the Fermi level (E ) of the surface. f
The theoretical approach of Terso and Hamann yields a tunneling current that only depends upon the sample DOS. However, their assumptions about the tip were unrealistic for most STM experiments. A more accurate account of the tip's e ect on the tunneling current and the STM image was developed by Chen.[16] Chen's theory, using an atomic orbital formalism, considers the e ect of di erent orbital overlaps
12 between the tip and sample on the STM scans. This theory helps to account for such things as varying sample corrugations and corrugation reversal (where the atomic sites appear as depressions in the STM image instead of protrusions). Although this theory considers various tip wavefunctions other that the s state, it is still a jellium calculation with many approximations. A more accurate examination of the tip and sample can only be calculated by rst principles. However, the shape and size of the metal cluster used to model the tip plays a very critical role in determining its calculated electronic properties.[19] While cut metal tips can yield atomic resolution, there are advanced techniques that allow more control over the nal structure of the STM tip. Since it is the purpose of this thesis to demonstrate a new technique for tip preparation, a brief review of the techniques that currently exist is necessary. However, none of these preparation techniques were used in these studies, so the reader is referred to more thorough reviews in the books by Bonnell and Chen. [20, 21] A rst step in tip preparation is an electrochemical etching of the metal tip which typical reduces the radius of curvature of the metal tip down to a few microns or smaller. For STM tips used in vacuum, e-beam and ion-beam bombardment are usually used to remove the contaminants built up during the etching process. Other techniques for cleaning the tip in situ include high voltage eld emission and intentionally crashing the tip into the substrate. A newer instrument, eld-ion scanning tunneling microscopy (FISTM), has the ability to view STM probes before and after scanning without removal from the STM head. [22] It also allows the shaping of the STM apex using high electric elds. However, it still does not allow one to view the tips in situ and is limited to
13 UHV conditions. A new technique that does allow observation of the STM tips during scanning by using defects on graphite as a surface tip array will be demonstrated in Chapter 3.
2.2.2 STM Imaging of Graphite Graphite has been used as a common calibration standard for STM because of its ease of preparation and the chemical inertness of its surface. A fresh graphite surface can be easily prepared by cleaving with adhesive tape. It has also been used as a substrate for the STM imaging of many molecular samples. In addition, graphite can be modi ed on the nanometer scale by voltage pulses from a STM tip. This has made it one of the most theorectically and experimentally analyzed surfaces with STM. The ideal crystal structure of graphite consists of stacked hexagonal planes of carbon with two inequivalent atomic sites as shown in Fig 2.2. The A site atoms on the surface are directly over another carbon atom, and the B site atoms are located directly over the center of a hexagonal ring, or hollow, in the next layer. This leads to an ABABAB... stacking sequence and is commonly referred to as hexagonal or Bernal graphite. The nearest-neighbor spacing in the plane is 1.42 A, while the interplanar distance is 3.35 A. The form of graphite most widely studied by STM, as well as in this thesis, is highly oriented pyrolytic graphite (HOPG). This polycrystalline material has grain sizes between 3 and 10 m. Approximately 5-15% of HOPG is in the rhombohedral form with an ABCABC... stacking sequence. However, most of the theoretical studies on the electronic properties of graphite are performed using the hexagonal crystal structure.
14
Figure 2.2: Crystal structure of hexagonal graphite The physical di erences between the A and B site atoms lead directly to an electronic di erence as well. The weak interaction of the A site atoms with the adjacent layers causes a decoupling between them and the B site atoms. This leads to a dispersion in the DOS of graphite so that only the B site atoms signi cantly contribute to the electron density at the Fermi energy. [23] Therefore the STM only images the B site atoms at low bias voltages creating an image like the one shown in Fig 2.3. Because only every other atom is imaged, the graphite lattice takes on a threefold symmetry with an atomic spacing of 2.46 A. However, it was demonstrated in ultrahigh vacuum (UHV) that at higher bias voltages the STM image of graphite will
ip to only imaging the A site atoms. [24] The observed asymmetry between the
15 atomic sites on graphite emphasizes that the STM images re ect the LDOS and not necessarily the surface atomic structure. Along with images of the normal graphite lattice, many anomalous images have been observed with STM and attributed to a variety of e ects. The most obvious source of anomalous images would be multiple tunneling sites on the STM tip. [25] A multiple tip can change the relative amplitudes and phases of the Fourier components that dominate the image of graphite, leading to triangular, elongated, or row-like images. Another anomalous image that is observed on graphite, and on metal surfaces, is corrugation reversal. [26] This occurs when a STM image switches from atoms appearing as protrusions to atoms becoming depressions and vice versa. These types
STM image of graphite obtained with a Pt/Rh Figure 2.3: A 90 A 90 A tip. The tunneling parameters were 3 nA tunneling current and -100 mV sample bias.
16 of images are expected when m 6= 0 states dominate the tip electronic states near the Fermi energy. For graphite, this yields a honeycomb image as shown in Fig 2.4. Depending on the tip orbitals that are present at the Fermi energy, the same distorted images that could appear with multiple tips may also be due to the symmetry of the dominate tip orbital of a single atom tip. [19] The sensitivity of the graphite images to di erent tip orbitals was also con rmed by Terso and Lang. [27] They concluded that the assumptions of Terso and Hamann do not apply when analyzing STM scans of graphite. Although, ideally the STM tip is not in contact with the sample surface, it has been shown that this is not always the case, especially when imaging graphite in air.
Figure 2.4: A 75 A 75 A STM image of corrugation reversal on graphite. 2 nA, -100 mV
17 An insulating contamination layer exists between the tip and sample which can cause compression of the graphite surface. [28] It is this force exerted by the tip that is thought to be the source of the unusually large corrugations of graphite that have sometimes been observed in air. In fact, even with point contact at the tunneling site, images of pristine graphite can still be observed. [29] Pethica explained these images as being due to changes in conductivity of sliding planes of graphite falling in and out of registry. [30] However, if nanometer-sized defects on graphite are imaged, then the STM image could not be due to sliding graphite planes. The observation of defects on graphite with both metal and fullerene-adorbed tips will be discussed in detail in Chapters 3 and 4.
2.3 Preparation of Fullerene STM Tips The preceding review of STM theory and tip preparation has made apparent the shortcomings of the standard metal STM probe. While beautiful atomic-scale images may be obtained with metal tips, many more images contain artifacts of the tip structure. Metal tips also have the drawback that they are dicult to theoretically calculate and predict their properties. On the other hand, molecularly functionalized STM tips o er the ability to create very reproducible probes with known electronic and structural properties. As mentioned in Chapter 1, fullerenes are very attractive for functionalizing STM tips. The following section describes how the fullerene-adsorbed STM probes are prepared. First, a thin fullerene lm is vapor deposited on freshly cleaved HOPG in high vacuum (10;6 Torr). The deposition rate is measured with a quartz crystal thickness
18 monitor and is typically between 1 and 2 A per second. The lm thicknesses range from 50 to 300 A. The apparatus used for growing the thin fullerene lms is described elsewhere. [31] The C60 in this lm is weakly bound to each other and the graphite substrate by van der Waals forces. After deposition these lms are then removed and transferred to the STM to create the fullerene-adsorbed tips. Once the fullerene thin lm sample is placed into the STM, the Pt/Rh (87/13) tip is approached into tunneling. The feedback circuit will move the metal tip through the fullerene lm until it reaches the graphite substrate. The tip is then rastered over the surface just as in the normal scanning procedure. The fullerene molecules bond to the metal tip by charge transfer which is much stronger than the van der Waals bond between the molecules. After a few minutes of sweeping the tip through the lm, it will become coated with fullerenes. As observed in previous STM studies of thin fullerene lms on metals, this bonding is strong enough to prevent the fullerenes from the rotation which is normally observed in bulk lms at room temperature.[32, 33, 34] This creates a relatively stable tunneling site on the apex of the STM tip to view the point defects. These tips are also very stable with respect to sample transfer. Since the fullerene tips require no special tunneling parameters, the samples can be scanned the same as with a metal tip.
19
Chapter 3 Inverse Scanning Probe Microscopy 3.1 Introduction As mentioned earlier, the scanning tunneling microscope (STM) and the atomic force microscope (AFM) have become essential tools for the observation of electronic and topographic structure of materials on the atomic scale. They have been used to investigate both crystalline and defect covered surfaces of various materials. The resolution of these techniques and their usefulness as accurate probes depend upon the structure of the tip. The convolution of the tip and sample is signi cant when the size of the tip becomes comparable to the surface feature size. The geometric shape of the tip is the most important factor in AFM since most AFM studies do not involve atomic resolution. The most crucial region of a STM is the nanotip where the tunneling occurs. Many studies have been done on the preparation and control of metallic scanning tunneling probes as mentioned in Chapter 2. Current electron microscopy methods often lack not only the resolution for imaging the nanotip, but, more importantly, cannot reliably locate the actual tunneling region. Other groups have attempted to use the STM technique to image standard STM tips, but the resolution was limited to tens of nanometers and there was no speci city for the STM tip tunneling site.[35, 36] Another technique, eld ion-scanning tunneling microscopy (FI-STM), allows monitoring of the tip structure before and after scan-
20 ning. [22, 37] Besides being limited to ultra-high vacuum, FI-STM tip images are nonlinear and re ect the state of the tip during eld evaporation and not during the scanning process.
3.2 Inverse AFM To view scanning probe tips in situ, Drexler proposed a method of imaging AFM tips with both molecular and nonmolecular tip arrays arranged on a substrate.[38] By varying the adsorbed molecules, the di erent forces between the tip and sample could be measured. This molecular functionalization of tips and substrates in AFM has already been demonstrated.[12] Using this technique, they were able to measure the forces between hydrophilic and hydrophobic functional groups in self-assembled monolayers (SAMs). A surface tip array composed of many di erent molecules would be able to analyze multiple interactions of a tip-adsorbed molecule with the substrate in one scan. No one has yet employed a molecular tip array in AFM, however, two groups have recently demonstrated the inverse imaging of standard AFM tips with di erent nonmolecular tip arrays. One group used aerosol deposited metal particles as a mask for dry etching of InP.[39] This technique produced surface columns with a diameter around 50 nm and a height of approximately 120 nm. The other researchers used copper thin lms prepared by metal organic chemical vapor depostion (MOCVD). [40] The dimensions of these surface tips were approximately 100 nm laterally and 300 nm vertically. When these surfaces are scanned, images of the AFM tip are generated by the tip arrays on the surface. Inverse AFM would not only allow di erent AFM
21 tips to be compared to each other, but would also aid in the deconvolution of tip e ects on other samples. While these structures work well for AFM tips, they fail to obtain a clear image of the tip apex and are therefore not appropriate for STM tips.
3.3 Inverse STM To achieve atomic scale resolution of a STM nanotip in situ, the secondary sites on the surface must be sharper than a normal STM tip. When this is the case, the STM scan re ects the image of the tip, not the surface. This e ect has been previously demonstrated with STM in the imaging of defect sites on semiconductor surfaces. [41, 42] Both groups could determine the metal tip structure from the tip artifacts created when the STM scanned over the surface defects. This study applies the same principle except that the surface defects are generated on graphite instead of semiconductors. Use of an inert surface like graphite is necessitated by the fact that the scanning is taking place under ambient conditions.
3.3.1 Defects on Graphite Many di erent techniques may be employed to generate defects on a graphite surface. Some that have been used previously include laser irradiation, chemical adsorption, chemical reaction, submonolayer epitaxial growth of metals, and ion bombardment. Each of these techniques is capable of creating defects with a wide range of sizes and morphologies. For this study we chose low energy ion bombardment because of the previous success shown in generating subnanometer defects on graphite.
22 Ion bombardment is an important tool for altering various properties of materials. It can be used to alter both the electronic and physical properties of the given surface. In the case of graphite, ion bombardment is used to increase the surface hardness of graphite. A sliding of the layer planes relative to each other gives unirradiated graphite its exibility. The defects induced by ion bombardment inhibit this sliding and increase the hardness. Since ion bombardment is limited to a few hundred nanometers in depth, the high bulk modulus, tensile strength, and high in-plane electrical conductivity of the bulk remain intact. Ion implantation can also be used to reduce the carrier mobility, thus changing the electrical properties of graphite. The scanning tunneling microscope provides the greatest spatial resolution over other techniques in analyzing the e ects of ion bombardment. It also allows the analysis of individual defects versus an ensemble averaged measurement such as Raman spectroscopy or Auger electron spectroscopy. STM can also measure the electronic changes induced by the ion damage, where atomic force microscopy is limited to measuring mechanical e ects. Because of graphite's two-dimensional layered structure, with only weak coupling between the layers, the structure and electronic properties of the defects revealed by the surface sensitivity of scanning tunneling microscopy will most likely resemble those in the bulk. Freshly cleaved highly oriented pyrolitic graphite (HOPG) was placed in vacuum at a base pressure of 10;9 torr. The samples were then bombarded with an ion uence of approximately 1012 argon ions per square centimeter. This dosage yields defect coverages that still leave most of the graphite surface intact. Argon was chosen over lighter ions because of its higher probability of generating defects upon impact. The
23 energy of the argon ions ranged from 40-90 eV. This low energy insured relatively small defects and damage con ned to the top few layers of the graphite surface. The low energy ion interactions with the surface can be treated as hard-spheres (shielded point charges) versus the electronic energy loss mechanism associated with high-energy ion bombardment. Bombarding graphite with low energy ions generates many di erent types of defects of the surface. Figures 3.1,3.2,3.3, and 3.4 show images of the di erent types of defects observed on graphite with a mechanically formed Pt/Rh (87/13) tip. Fig 3.1 is an image of a point defect on graphite. Fig 3.2 also shows a point defect but with
p
p
an extended perturbation of the graphite lattice. This 3 3 R30 superlattice is an electronic perturbation in the vicinity of the defect and not an actual physical reordering of the graphite lattice. [43] A subimplantation, or \dome", defect is shown in Fig 3.3. This is caused by the trapping of an argon ion between the uppermost two layers of the graphite surface without introducing any interstitial or vacancy defects into the lattice. [44] Fig 3.4 is an amorphous defect, the interpretation of which is more dicult. It appears that many carbon atoms are displaced in this defect, but an 80 eV argon ion only has enough energy to displace 4 atoms at the most. A combined AFM and STM study of low energy ion damage to graphite determined that the large size of the defect is really an electronic e ect. [45] It is due to vacany-induced charge enhancement and is therefore essentially a tip artifact of scanning graphite defects with a metal STM tip. It is not a physical tip artifact since the same tip also imaged Fig 3.1.
24
Figure 3.1: A 45 A 45 A STM image of a point defect on graphite. 2 nA, -100 mV
Figure 3.2: A 90 A 90 A STM image of a superlattice defect. 1 nA, +200 mV
25
Figure 3.3: A 60 A 60 A STM image of a subimplantation defect. 4 nA, -100 mV
Figure 3.4: A 77 A 77 A STM image of an amorphous defect. 2 nA, +100 mV
26 In some instances, the graphite defects can be used to analyze the tunneling site of a metal tip. Fig 3.5 shows a STM image of graphite using a metal tip where at every defect site an inverse image of the tip is generated. The di erences in the inverse images are due to the varying radii of curvature of the defects. However, in general, the defect sites appear unique and no clear inverse image of the tip can be discerned. It is therefore necessary to use caution in interpreting the STM images of graphite defects with bare metal tips. Images of the defects can only be obtained with a sharp STM tip, and even then, one must be careful in trying to determine what part of the defect image is due to topography and what is due to the electronic nature of the defect site.
Figure 3.5: A 600 A 600 A Inverse images of a bare Pt/Rh STM tip. 1 nA, +100 mV
27
3.3.2 Inverse STM of fullerenes In this section, the inverse imaging technique is applied to the analysis of fullereneadsorbed STM tips. As discussed in Chapter 2, rastering a Pt/Rh tip in a thin fullerene lm results in the adsorption of fullerene molecules onto the tip and improves the atomic imaging of graphite. Because this is done under ambient conditions, there are many possible explanations for the improved atomic resolution. Analysis of the role of the adsorbed fullerene molecules in this e ect is dicult by conventional techniques for many reasons. There is less than a monolayer of molecules that are adsorbed onto the tip. The area of the tip that is the actual tunneling site is not obvious in scanning electron micrographs of the tip. Finally, the fullerene molecules may be somewhat mobile on the tip since they are not covalently bonded to the metal, but instead, undergo charge transfer. Using graphite defects as a tip array enables the e ect of the fullerenes on the imaging mechanism to be studied in situ. In addition, the known structure of the fullerenes allows for an easier interpretation of the inverse images, as compared with a bare metal tip. Fig 3.6 is a large area scan of a defect-covered graphite surface imaged with a fullerene-adsorbed tip. At each defect site in the scan, a group of three C60 molecules can clearly be seen. The molecular arrangement is identical at each defect site, indicating that the defects are imaging the molecules on the STM tip. The di erence in contrast between each inverse image is indicative of the imaging di erences between each defect site. In general, the arrangement of molecules on a tip was stable during a single image acquisition (30 sec) but showed evidence of molecular rearrangement over a time period of several minutes. Figures 3.7 and 3.8 show two consecutive
28 scans where the structure of the fullerene tip has changed during imaging. It also demonstrates the di erences in inverse imaging due to the varying radii of curvature of the defects. Typically, the fullerene molecules could be imaged in the tunnel junction for hours. Figures 3.9 and 3.10 show inverse tip images at higher resolution than in the previous gures. In Fig 3.9, two fullerene molecules are imaged on this tip. The di erence in contrast indicates that one fullerene molecule is positioned signi cantly closer to the surface than the other molecule. Of interest in these images is the clear appearance of atomic resolution on the graphite substrate surrounding the secondary tip. Typically, when a single or predominant fullerene molecule is imaged on the
scan with inverse images of a fullereneFigure 3.6: A 220 A 220 A adsorbed STM tip. 1 nA, +100 mV
29
Figure 3.7: A 200 A 200 A scan of a three fullerene tip. 1 nA, +100 mV
Figure 3.8: Inverse images of the same tip approximately one minute later. Same scan parameters
30 tip, clear atomic resolution on graphite is observed. The presence of several fullerene molecules of equivalent height usually corresponds to little or no resolution of the underlying graphite lattice. This indicates that the fullerene molecules are functioning as true tunneling sites, a single molecule corresponding to a \good" tip with a single tunneling site and a collection of equivalently placed molecules corresponding to a \poor" tip with multiple tunneling sites. The appearance of inverse images of the fullerene tip in the same scan as atomically resolved graphite lattice indicate that the improved resolution of graphite is probably due to tunneling through the fullerene molecule. This low voltage tunneling through a C60 molecule is consistent with previous STM observations.[46] The inverse
Figure 3.9: A 65 A 65 A image of two fullerenes on adsorbed on a tip. 1 nA, -100 mV
31
Figure 3.10: A 80 A 80 A scan of a single fullerene on the tip. 1 nA, +100 mV imaging process also reveals the internal structure of the fullerene molecules. The intramolecular structure of the fullerenes resolved with inverse imaging demonstrates that this technique seems to work about as well as standard STM.[34, 47] The mobility of the fullerenes observed during inverse imaging demonstrates how tunneling conditions can change between scans and how subtle these changes can be without a sensitive tip diagnostic tool. This demonstrates that at some of the defects sites the apices of the defects protrude from the surface and function as secondary tips to generate inverse images. Even though some of these defects are larger than a few angstroms, their sharp apices make them good surface tips for inverse imaging. Since defects generated on graphite by ion bombardment have been studied previously,[48, 49, 50, 44, 51] this
32 technique allows good control over the both the number and the morphology of the defects on the surface. Another advangate is that these defects are stable in air for weeks. However, not all of the defects on graphite observed with a fullerene-adsorbed STM tip generate inverse images. At many of the defect sites, the fullerene tips show patterns of electron scattering around the defects. The nature of the electron scattering images and the inverse images observed wihen tunneling through a C60 molecule are the subject of the next chapter. The above results demonstrate a method of imaging individual molecules which have been adsorbed onto a Pt/Rh STM tip. Defect sites created on a HOPG surface by low energy ion bombardment serve as secondary tips and are responsible for the inverse images of the tip-adsorbed molecules, revealing intramolecular contrast in the individual fullerene molecules. [34] This also demonstrates that the improved resolution of graphite with fullerene-adsorbed STM tips is most likely due to tunneling through the fullerene. With the success of this technique, further studies involving the oriented attachment of functionalized fullerenes to STM or STM/AFM tips, their chemical modi cation and subsequent imaging, and the role played by fullerenes in tunneling are now possible. This imaging method should also prove useful in the characterization of tunneling sites on either conventional or modi ed STM tips.
33
Chapter 4 Electron Scattering on Graphite 4.1 Introduction Since the STM images the electronic structure, as well as the topography, of the surface; it has enabled scientists to view local surface electronic phenomena on the nanometer scale. Some examples include localized defects in charge density waves, [52] superconducting vortices, [53] and the resistance of a single C60 molecule. [13] In addition, various groups have been able to image electron scattering phenomena from defects on metal surfaces through the use of low-temperature STM,[54] spatial dI/dV spectroscopy,[55] and thermopower microscopy.[56] These standing wave patterns are formed by the scattering of surface state electrons from point defects and steps. One of the rst systems predicted to have electron scattering observable with a STM was a semimetal, graphite. [14]
4.2 Theory of Electron Scattering on Graphite Mizes and Foster calculated theoretical STM images of electron scattering that would accompany various point defects and strongly bound adsorbates on graphite. Their calculations are based on the disruption of the electrons at the Fermi surface induced by the presence of point defects. This local perturbation of the charge density by the defect can lead to periodic oscillations similar to Friedel oscillations. This gives
34 the defect the appearance of a three pointed \star" re ecting the threefold symmetry of the graphite about the defect. This is an electronic modulation and not a reconstruction of the graphite lattice. The symmetry of these oscillations was also shown to re ect the symmetry of the defect and thus yield possible identi cation of the adsorbate. Yet, Mizes and Foster did not experimentally observe these images and neither
p p
did prior and subsequent experiments. Instead, only 3 3 superlattices have been observed in the vicinity of some defects on graphite,[14, 57, 43, 49] as in Fig 3.2 in Chapter 3. Recently, Kondo et al. reported a threefold image due to a single atom vacancy on graphite. [58] But this image bears no similarity to the images calculated by Mizes and Foster. Subsequent papers have been published to attempt to explain the discrepancy between the observed images and theory using revised scattering theories.[59, 60] However, as is shown in the next section, the theory of the scattering on the graphite surface need not be changed. Instead, it is the role of the tip in viewing the electron scattering that needs to be revised. All three scattering papers used Terso and Hamann's approximations in the modeling of the STM tip. [18] As previously mentioned, this was shown to be invalid when imaging graphite with the STM. [27] The scattering images predicted by Mizes and Foster are clearly visible when a fullerene molecule is adsorbed onto the tunneling region of the STM tip.
35
4.3 Imaging with C60 Tips Chapter 3 illustrated the in situ observation of the fullerene-adsorbed STM tips using inverse imaging techniques.[61] With the same technique, we are also able to observe the threefold scattering near point defects on graphite. This functionalization of STM tips with fullerenes to control the electronic interactions between the tip and sample is similar to the attachment of molecules to AFM tips by other researchers to control the atomic forces between the tip and sample.[12] Instead of the chemical forces, however, the STM tunneling current is more sensitive to the density of states (DOS) at the Fermi energy (E ) of the tip and sample. This e ect has already been f
demonstrated in the study of metal oxides by alternating apex atoms of the STM tip between tungsten and oxygen.[62] By altering the DOS of the tip with a C60 molecule, the electronic states of the sample that can be observed by the STM are changed.
4.3.1 Experimental Results The fullerene-adsorbed STM tips were created as previously described in Chapter 2. The point defects were created by low energy argon ion bombardment of freshly cleaved HOPG as mentioned in Chapter 3. Using a Pt/Rh tip we analyzed the ion damaged samples to con rm that the size and uence of the ions agreed with previous experiments. When these samples were scanned with a bare metal STM tip, point defects with and without the superlattice were observed. Subsequent scanning with the fullerene coated STM tips revealed inverse images of the fullerenes and the threefold electron scattering images.
36 Figures 4.1 and 4.2 are two experimental images of defects on graphite taken with a C60 molecule attached to the STM tip. Figures 4.3 and 4.4 show two of the theorectical images of electron scattering predicted to accompany point defects on graphite.[14] Both are multiple point defects with the only di erence being in the placement of the defects with respect to the graphite lattice. The agreement between the observed and the calculated defects is excellent, including the similarity between the superlattice structures. The superlattice is expected to be indicative of the central defect morphology.[14] These oscillations are very similar to those observed by STM of metal surfaces. However, on metal surfaces the electrons scatter symmetrically around the defect, whereas on graphite the electrons primarily scatter in three crystallographic directions along the graphite surface. The spherical nature of the center of the threefold defects is probably due to an inverse imaging of the fullerene tip by the central apex of the defect. Fig 4.5 shows a series of line scans taken along one of the scattering arms of a threefold defect at di erent biases. As the absolute value of the bias voltage is increased the length of scattering arms from the defect decreases as does the amplitude of the oscillations. Also, the superlattice between the arms of the defect disappears as bias voltage is increased. Without theoretical calculations, it cannot be determined if this e ect is due to the electronic structure of the defect or the width of the resonance of the fullerene tip. Because the amplitude of the graphite corrugation is approximately the same for all linescans, it is not an artifact of imaging the graphite in air. As Fig 4.5 clearly illustrates, the wavelength of the electronic oscillations that give rise to this scattering pattern does not vary with bias voltage, unlike the oscillations
37
Figure 4.1: Electron scattering on graphite observed with a C60 tip. 60 A 60 A ,1 nA, -100 mV
Figure 4.2: Electron scattering on graphite observed with a fullereneadsorbed STM tip. 85 A 85 A ,1 nA, +100 mV
38
Figure 4.3: Theoretical image of electron scattering from Mizes and Foster.[14]
Figure 4.4: Theoretical image of electron scattering from Mizes and Foster.[14]
39 observed in the scattering of electrons in surface states on metal surfaces.[54, 55] This, along with the threefold directionality of the scattering, is another important di erence between electron scattering observed on metals and on graphite. In contrast to the free-electron-like surface states on metals, the electrons at E on graphite are f
tightly bound to the atomic sites. This results in oscillations that maintain the periodicity of the B site trigonal sublattice, as well as the anisotropic scattering patterns observed. In addition to observing electron scattering phenomenon near point defects on graphite, we have also observed pronounced periodic electron scattering near graphite steps as shown in Fig 4.6. Previous investigations of graphite have reported the imag-
p
ing of 3 superlattice structures near steps and grain boundaries on graphite;[57, 63] 3
2.5 +0.3 V 2
1.5 Tip height (nm)
+0.1 V 1
0.5 −0.1 V 0 −0.3 V −0.5 −0.5 V −1
−1.5 0
1
2 3 Distance (nm)
4
Figure 4.5: Depedence of threefold scattering on tip bias voltage.
40 however, pronounced periodic electron oscillations have not previously been observed. These oscillations appear stronger on the upper terrace of the step, similar to the electron scattering from steps imaged on metal surfaces.[54, 55] However, unlike metal surfaces, the scattering does not occur in plane waves propagating perpendicular to the step. Instead, the scattering seems stronger along the directions of the graphite lattice, as seen in the scattering around the point defect shown in the gure. The electronic perturbation caused by the step persists for tens of angstroms before attenuating to the unperturbed lattice.
Figure 4.6: A 100 A 100 A image of electron scattering from a graphite step. 2 nA, +100 mV
41
4.3.2 Imaging Mechanism With fullerenes adsorbed onto the STM tip, we observed that most defects were either spherical in nature or showed a threefold symmetric extended structure. The di erences between the spherical and the threefold images can be explained by the theory of Mizes and Foster. Both images can be considered to be generated by point defects, but the observed STM images depend on the location of the defect relative to the graphite lattice. The graphite surface is composed of a two-dimensional hexagonal lattice of carbon atoms. The layers are staggered in the three-dimensional crystal creating two inequivalent sites on the graphite surface. The A;site carbon atom is directly above an atom in the second layer, while the B ;site carbon atom is above a hollow. This inequivalence alters the electronic nature of the two sites, and consequently the STM images only the B ;site atoms of the graphite lattice at low bias voltages.[23, 24] Therefore, electron scattering images are not expected to be observed by the STM from a single bond made to an A;site atom.[14] In addition, electron scattering images are not expected due to possible phase cancellation between nearby scatters on either site of the graphite lattice.[14] Without the surrounding superlattice, the defect is a point defect; and with its sharp aspect ratio, inverse images the fullerene on the STM tip.[61] It is also probable that the occurrence of inverse images at all defect sites is due to multiple layers of fullerenes adsorbed to the STM tip based on the following reasons. First, it is extremely unlikely that all defects in a single image are A-site defects. Also, the imaging of the superlattices depends on the charge transfer between the tip and fullerene. Subsequent layers of C60 would have a much reduced charge transfer from
42 the platinum, if any at all. The charge transfer is also extremely localized to one side of the C60 as shown by molecular orbital calculations on organometallic derivatives of C60 . [64] Experimentally, multiple inverse images are seen at the beginning of scanning, while after a short period of scanning with the fullerene tips, the prevalent images are threefold electron scattering. This would also be indicative of initial multiple layers of fullerenes adsorbed to the STM tip. Then, as the STM scans, the more loosely bound layers of C60 are removed, leaving a single monolayer strongly bound to the metal tip. Adsorption of a C60 molecule to the STM tip tunneling region changes the local density of states of the tip. Fullerenes strongly chemisorb to most metals and undergo charge transfer from the metal to the adsorbate molecule. This shifts the lowest unoccupied molecular orbital (LUMO) of the C60 towards E of the metal. This f
shifting of the fullerene LUMO level has been observed by photoemission and inverse photoemission of C60 thin lms on various metals.[65, 66] The molecular adsorbate thus creates a partially lled molecular level at the apex of the tunneling tip near E . It is possibly this molecular adsorbate level, which is narrow in energy compared f
to the bulk bands of the tip, which enables the fullerene-adsorbed tip to resolve the electron scattering at E of graphite when bare metallic tips cannot. Because the f
LUMO level of the fullerene is only partially lled, the threefold images are observable at both positive and negative bias voltages. A narrow tip local density of states can also explain why these electron scattering patterns could be imaged in topographic (rather than dI/dV) mode.
43 Not only does the fullerene molecule adsorbed on the STM tip modify the tip electronic properties, it can also a ect the tunneling process by changing the wavefunction overlap between the tip and the sample. The changes in orbital overlap lead to changes in the tunneling matrix element, which greatly in uences the STM image. The in uence of di erent orbitals in STM has been discussed in great detail by Chen.[67] Chen showed that a p -p overlap gives rise to larger atomic corrugation z
z
in a STM image than p -s, and that p -d gives rise to the largest corrugation. It z
z
z
would follow that this is the order of greater orbital-orbital overlap, and the best wavefunction overlap would typically be obtained on graphite surfaces with metal tips. However, less than 25% of the d electrons at E of platinum are d in nature, f
z
while the other electronic states of platinum have central nodes that would decrease the orbital-orbital overlap on graphite relative to the p -d orbital overlap.[68, 26] z
z
On the other hand, the electron orbitals in graphite and C60 that participate in the tunneling process are all p in nature. The e ect of di erent tip orbitals has also z
been calculated speci cally for the case of the graphite surface.[27] It was shown that di ering tip states have a dramatic e ect on the atomic scale images of the graphite lattice. This could also explain the enhanced resolution of a fullerene-adsorbed STM tip as compared to a platinum STM tip and why, even at low bias voltages, STM images of defects on graphite can vary so drastically between metal and fullerene tips. These results demonstrate that by adsorbing fullerene molecules to the apex of a STM tip it is possible to observe the predicted electron scattering from point defects on graphite. This phenomenon does not appear when the graphite is scanned with a bare metal STM tip. It is probable that the threefold scattering is imaged due
44 to tunneling through the molecular levels of the fullerene molecule. We have also observed a similar change in STM images of electron scattering from steps on graphite. This demonstrates the feasibility of molecular functionalization of STM tips to control the electronic interactions between the tip and sample, and the possibility of imaging di erent combinations of electronic states.
45
Chapter 5 Future Directions 5.1 Introduction This research has demonstrated that the measuring process in STM is not always straightforward. Because of the resolution that is achievable, the probe must be considered an intricate part of the system. It has also been shown that molecular functionalization of STM probes allows one to choose the electronic interaction between the tip and sample. But there are still questions to be answered with this research. Here is a summary of possible experiments that may aid in answering these questions.
5.2 Inverse Imaging Inverse imaging of the tip with graphite defects o ers many advantages over conventional techniques. It is an in situ technique that re ects the state of the STM tip during tunneling. Defects on graphite can be imaged on the atomic scale in air or in solution. In the cases of organic and electrochemical STM, a technique is needed to examine the tip under tunneling conditions. Adsorbates that might interfere while the STM is tunneling could be removed by the high vacuum or high electric eld conditions which current analysis techniques require. Since graphite is a common substrate in organic and electrochemical STM, these defects can be placed directly
46 on the substrate. The many studies already done on graphite defects provide a wealth of knowledge about the topographic structure of the defects.[48, 49, 50, 44, 51] This technique may also allow observation of changes to the tip by various cleaning methods like eld emission or tip crashes.
5.3 Electron Scattering on Graphite The defects used to observe electron scattering all have di ering morphologies. For a more thorough investigation, a uniform defect structure is needed. This could be accomplished by chemisorption, as in the original experiments by Mizes and Foster. [14] It may also be possible to observe electron scattering from graphite substitutionally doped with boron and nitrogen. It would also be interesting to compare the di erences in chemisorbed and physisorbed defects with a fullerene-adsorbed STM tip. The electron scattering could be a measure of the strength of interaction of the molecule with the graphite. Seeing how the fullerene tips image charge density waves on graphite intercalation compounds may also lead to a greater insight into the electronic di erences between fullerene and metal STM tips. The electron scattering images themselves could be used as a tip diagnostic tool similar to the inverse imaging defects. By comparing experimental images of threefold scattering from various tips to the theory, it may be possible to gain information about the tip molecular orbitals involved in the tunneling process. C60 adsorbed to di erent metal tips should yield di erent scattering images based on previous studies of the charge transfer from metals to fullerenes. [65, 66] A comparsion of the e ects on the LUMO and HOMO levels between di erent chemical modi cations to C60 could
47 be made. Some of the more interesting comparisons might be between ourinated, hydrogenated, and endohedrally doped C60 . Also, by comparing the changes in the scattering images as the carbon cluster size is increased may yield information about the relative molecular levels and how this a ects charge transfer.
5.4 Fullerene STM Tips There are many options available for further studies with the fullerene-adsorbed STM tips. A rst step would be to move the experiments to UHV which would provide a cleaner and more controlled enviroment. This should improve the stability of the C60 molecules on the tip. For more stable fullerene tips in air, the Langmuir-Blodgett and self assembly techniques that are already available with fullerenes could be used. These techniques would creat a stable uniform monolayer on the STM tip. Fullerene tips could also be applied to other layered materials such as dichalcogenides, superconductors, or any material with sharp features in the DOS near E . Lastly, fullerenes f
could be adsorbed to ferromagnetic STM tips. Presumably, the electrons in the partially lled LUMO level would become polarized because of the polarized levels at E of the tip. This may create a spin-polarized STM probe with high spatial and f
energetic resolution. Another interesting avenue would be to use the fullerene-adsorbed tip as one half of a device in the STM junction. Other reseachers have already measured the I(V) characteristics for a single C60 molecule. [46] Using a fullerene-adsorbed STM tip and a molecule adsorbed on a substrate electrode, it should be possible to measure the I(V) characteristics of a two molecule device, ie. C60 and MEH-PPV. This type
48 of local measurement could then be used to directly compare electronic interactions between molecules.
49
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