Temperature dependence of current transport in metal-swnt structures

Transcription

1 The University of Toledo The University of Toledo Digital Repository Theses and Dissertations 2015 Temperature dependence of current transport in metal-swnt structures Robert Daine University of Toledo Follow this and additional works at: Recommended Citation Daine, Robert, "Temperature dependence of current transport in metal-swnt structures" (2015). Theses and Dissertations This Thesis is brought to you for free and open access by The University of Toledo Digital Repository. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of The University of Toledo Digital Repository. For more information, please see the repository's About page.

2 A Thesis entitled Temperature Dependence of Current Transport in Metal-SWNT Structures by Robert Daine Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Physics Dr Michael Heben, Committee Chair Dr Richard Irving, Committee Member Dr Lawrence Anderson-Huang, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo August 2015

3 Copyright 2015, Robert John Daine This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

4 An Abstract of Temperature Dependence of Current Transport in Metal-SWNT Structures by Robert Daine Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Physics The University of Toledo August 2015 Single walled carbon nanotubes (SWNTs) have been under the microscope recently due to their incredible and unique mechanical, electrical, and optical properties, which are influenced by their chirality and diameter. Many different applications have been looked into, such as nanotechnology, electronics, and biomedical applications. Recently, it has been suggested that SWNTs may act as a tunnel between the p-n junction in a solar cell. In this thesis, the temperature dependence of the activation energy between SWNTs and metal electrodes was looked at, using a mixture of gold, aluminum and copper electrodes. Because we formed a Schottky barrier between the semiconducting SWNTs and the metal electrode, we know that the decrease in activation energy allows the electrons and holes to travel quicker and easier than at higher temperatures. iii

5 Acknowledgements I wish to thank my advisor, Dr Michael Heben, for all of his help and guidance through the process of getting this thesis done. For helping me find a project that was interesting and insightful. I would also like to thank Dr Adam Phillips for his constant support and feedback, Dr Rajendra Khanal and Suneth Wattage for their constant help with nanotube growth and synthesis stages. I also want to thank the rest of Dr Heben's research group and Dr Ellingson's group, including Paul Roland, Dr Neale Haugen and Niraj Shrestha for all of their help leaning how to use the vacuum pump and the cryostat. Finally, I want to thank my fiancée Markie Miller for her constant love and support throughout this process. v

6 Table of Contents Abstract... iii Acknowledgements...v Table of Contents... vi List of Tables... ix List of Figures...x List of Abbreviations... xii 1 Motivation and Overview of Thesis Uses of Single-Walled Nanotubes Overview of Thesis Carbon Nanotubes Introduction Structure of Single-Walled Nanotubes Electronic Properties of SWNTs Characterization of Multi-Walled Nanotubes Synthesis and Characterization of SWNTs Introduction Growth of SWNTs Synthesis of SWNTs Arc Discharge - AD...32 vi

7 3.3.2 Laser Vaporization - LV Chemical Vapor Deposition - CVD CoMoCAT Method Purification of SWNTs Fabrication of Films Preparation of Cells Deposition Techniques Membrane Transfer Method Drop Casting Film characterization X-Ray Diffraction Electron and Atomic Force Microscopy Ultraviolet to Near Infrared Spectroscopy Raman Spectroscopy Four Point Probe Method Applications of SWNTs Low Temperature Measurements on SWNTs Introduction Cryostats Closed-Cycle Cryostats Components of the Closed-Cycle ARS DE Cryostat The Gifford-McMahon Refrigeration Cycle Measurements...60 vii

8 4.2.5 Continuous-Flow Cryostats Bath Cryostats Multistage Cryostats Data Acquisition Conclusion and Future Discussion CdTe Solar Cells Introduction Results and Discussion...76 References...78 A LabVIEW Program...83 viii

9 List of Tables 2.1 The physical properties of different carbon allotropes Parameters for zigzag and armchair nanotubes Work functions and electron affinities of metals used Comparison of SWNTs and MWNTs Values for the correction factor Calculated sheet resistance, resistivity and conductivity for each temperature Schottky barrier heights of both laser and CoMoCAT produced SWNTs...69 ix

10 List of Figures 2-1 An image of a layer of graphene Different types of nanotube Hexagonal lattice of graphene with first Brillouin zone D image of graphene Energy band diagrams for different types of material Energy band diagram - the barrier between a metal and semiconductor Density of States for armchair and zigzag nanotubes Russian Doll model of MWNTs Growth mechanisms for SWNTs: tip-growth and base-growth method Schematic diagram of the arc discharge system A fluidized bed reactor used in the CoMoCAT method Vacuum filtration and transfer method of SWNTs XRD of laser produced SWNTs Microscopy images of SWNTs UV-Vis spectroscopy of SWNTs Raman Spectroscopy of LV and CoMoCAT SWNT films Collinear arrangement of the four point probe Finding the correction factor for different thickness-spacing ratios Four point probe measurements for CoMoCAT SWNTs...53 x

11 4-1 Diagram of set up and components of the ARS DE closed-cycle cryostat The GM refrigeration cycle Comparison of LV-synthesized and CoMoCAT SWNTs Asymmetrical contacts, showing asymmetry by differentiation Current versus 1/T for symmetrical contacts with laser SWNTs Current versus 1/T for symmetrical contacts with CoMoCAT SWNTs Current versus 1/T for asymmetrical contacts with CoMoCAT SWNTs Conductivity versus 1/T for samples with symmetrical contacts Exponential curves of ln(σ) versus 1/T Linear plot of barrier height versus temperature - symmetrical electrodes Linear plot of barrier height versus temperature - asymmetrical electrodes Superstrate configuration of CdTe solar cells Comparison between CdTe solar cells...77 A-1 The Front Panel of the LabVIEW Program...83 A-2 First section, temperature setting and measurement...84 A-3 The Set Temperature Sub-VI...85 A-4 Read the temperature on probe A...86 A-5 Check if the temperature is in range...87 A-6 Settling stage...88 A-7 Record current and voltage...89 A-8 J-V measurement sub VI...90 A-9 Tabulate the data...91 xi

12 List of Abbreviations AD...Arc Discharge CdTe...Cadmium Telluride CoMoCAT...Cobalt Molybdenum Catalyst CVD...Chemical Vapor Deposition DI...De-ionized Water ev...electron Volt FET...Field Effect Transistor IR...Infra-Red I-V...Current-Voltage LV...Laser Vaporization MWNT...Multi-Walled Nanotube nm...nanometers PV...Photovoltaics SBH...Schottky Barrier Height SDBS...Dodecyl Benzene Sulphate SDS...Sodium Dodecyl Sulphate SEM...Scanning Electron Microscopy SWNT...Single Walled Nanotube TCO...Transparent Conducting Oxide TEM...Tunneling Electron Microscopy UV...Ultraviolet XRD...X-Ray Diffraction xii

13 Chapter 1 Motivation and Overview of Thesis 1.1 Background Semiconductor materials are the backbone of all modern electronic devices and are used in the manufacturing of solar cells, diodes and many different digital and analog integrated circuits [1]. One of the semiconducting materials being focused on more recently, and throughout this work, is carbon nanotubes; more specifically single walled carbon nanotubes. Currently, silicon is used in abundance in many electronic devices, though it is not cheap to fabricate or process Si devices at the large scale, so better options are needed. Since their discovery in 1991 [2], SWNTs have been found to have unique structural and electronic properties. The one dimensional properties of SWNTs have been used in applications such as electronics, photovoltaics, batteries [3], and in medical treatments and devices [4]. Because of the potential applications in PV technology, this thesis will look at the application of SWNTs at a tunneling junction between two materials. Different characterization techniques were used to further understand the properties of 1

14 the SWNTs and a cryostat was used to see the effect on the Schottky barrier height at low temperatures. 1.2 Overview of Thesis This thesis is organized in the following way Chapter 2 will delve into the background and theory of single walled nanotubes, in terms of the structural properties and their electronic properties, such as band structure and density of states. Chapter 3 will discuss the different methods of synthesis for SWNTs, as well as the purification and fabrication of thin films and samples used. Chapter 4 will talk about the data collection methods used and the results from each one. The theory of the four point probe method and the assumptions we must make to take accurate data will be discussed. Chapter 5 will summarize this thesis and discuss any future work inspired by the work done here. Chapter 6 is an additional chapter, which will discuss the use of SWNTs as a tunneling layer between the p-n junction in CdTe solar cells. Appendix A includes the LabVIEW diagrams showing how the data was collected from the cryostat using a Keithley 2401 Sourcemeter and a Lakeshore 335 Temperature Controller. 2

15 Chapter 2 Carbon Nanotubes 2.1 Introduction Carbon nanotubes were first discovered by using tunneling electron microscopy to look at soot synthesized by the arc discharge method [2]. Carbon nanotubes are one of the many allotropes of carbon. There are two different types of nanotube; single-walled nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). Figure 2-1: An image of a layer of graphene with atoms labeled with the indices (n, m) and unit cell base vectors and [5]. 3

16 By visualizing SWNTs as being made of one-atom thick layers of graphene, it can be seen that they have a cylindrical structure, which in the single-walled case, are constructed in different ways [6]. They have many uses in industry, but are difficult to obtain separately. SWNTs show a combination of unique physical properties such as strength and stiffness, that no other material does at the same time. Similarly, the thermal and electrical conductivities of SWNTs are higher than others. SWNTs have a density of g/, a thermal conductivity of 200 W/mK, an average tensile strength of roughly 100 GPa, a maximum current density of approximately A/m², and a Young s modulus of around 1 TPa [7]. 2.2 Structure of Single-Walled Nanotubes As mentioned earlier, SWNTs can be visualized as being a flat sheet of graphene, which is then wrapped in a cylindrical fashion. The way it is wrapped actually determines the properties of the nanotube itself, this is represented by a pair of indices. These are used to indicate the number of unit vectors along both directions in a 2 Dimensional axis of the honeycomb crystal lattice. The structure of these nanotubes can easily be specified in terms of the lattice vector,, which joins two equivalent points on the grapheme lattice, which when rolled, then superimpose. The integers (, ), also known as chiral indices, represent a possible tubular structure which can be expressed as [8] (2.1) 4

17 Where and are the unit cell base vectors of the sheet of graphene and is always true [9]. There are three types of SWNTs; Armchair (as seen in Fig 2.1 (a)), Zigzag (as seen in Fig 2.1 (b)) and Chiral (as seen in Fig 2.1 (c)). Figure 2-2: Here is an Armchair nanotube, which is wrapped with the indices n=m and has a chiral angle of 30, a Zigzag nanotube, which is wrapped with the indices m=0 and has a chiral angle of 0, and a chiral nanotube which has a variable chiral angle [10]. 5

18 The length of an ideal nanotube can be calculated as (2.2) from here, the diameter can be calculated as (2.3) Where a is found to be (2.4) where is the carbon-carbon bond length. The chiral angle of the nanotube can also be found by any one of the following [8] (2.5) The diameter of SWNTs is limited by the energy needed to keep the tubular shape. Theoretically, the smallest possible SWNT is the (5,0) zigzag nanotube with a diameter of 0.39 nm, the (3,3) armchair nanotube with diameter of 0.41 nm, and the (4,2) chiral nanotube with a diameter of 0.41 nm. The diameter of a typical nanotube is approximately nm. Another convention characterizes C by two integers so, where the primitive translation vectors are and. here, the corresponding nanotube is referred to as a nanotube. These integers are related to n and m by and. This means. 6

19 A primitive translation vector in the y-axis is given as (2.6) with integers and. Because the chiral vector is in the x-axis, we know that must be true, this can be rearranged such that (2.7) which can be solved as (2.8) where is the most common divisor of both and and (2.9) where d is the greatest common divisor of (n,m). The first Brillouin zone is given by the region with the length of the translation vector as (2.10) The unit cell is created by a rectangular region determined by and. The number of atoms in this cell can be found by (2.11) and the number of hexagons in this cell is (2.12) 7

20 The chemical bonding of nanotubes is composed of sp² carbon-carbon bonds. This bonding structure is stronger than the sp³ bonds in diamond and gives the nanotubes high mechanical properties. It is well documented that the mechanical properties of nanotubes exceed those of other materials. Even though there is no unanimity on the precise mechanical properties, theoretical and experimental results have shown unusual mechanical properties of nanotubes, with an extremely high Young's modulus value of 1.2 TPa and tensile strength of GPa. These properties make carbon nanotubes the strongest material on earth. As well as the incredible mechanical properties exhibited, the carbon nanotubes also possess other useful physical properties, as shown in table 1. It's obvious that carbon nanotubes, both single- and multi-walled, have many advantages over the other allotropes of carbon in terms of electrical and thermal properties. These properties offer great potential in many applications, as discussed in section Table 2.1: The physical properties of different carbon allotropes [11]. Carbon material Property Graphite Diamond Fullerene SWNT MWNT Specific gravity (g/cm³) Electrical conductivity (S/cm) Electron mobility (cm²/(vs)) Thermal conductivity (W/(mK)) Coefficient of thermal expansion ( ), Negligible Negligible Thermal stability in air ( C) 8

21 2.3 Electronic Properties of SWNTs Before we can learn and understand the electronic structure of nanotubes, we must first learn about graphene. As we saw in figure 2-1, graphene consists of a hexagonal lattice of carbon atoms. These atoms all have 4 valence electrons, 2 of which are in the s-state and two in the p-state, and three of them are hybridized into sp² orbitals, while the orbital remains perpendicular to the other orbitals [12]. Each of the sp² atoms form a strong covalent bond with neighboring carbon atoms, this is known as a σ- bond, and the electron undergoes π-bonding. In general, the π orbitals (near the Fermi level) are generally responsible for the electrical transport properties, by forming delocalized states [13]. The lattice vectors can be written in real space as (2.13) (2.14) where, and is the distance between the two carbon atoms, as before. Every real lattice vector has its own unique reciprocal lattice vector. The primitive cell in reciprocal space is known as the first Brillouin Zone (BZ). This is the only region in which we consider energy dispersion calculations. This reciprocal space is also known as k-space and the vector which locates any point inside the BZ is known as k, the wave vector. Just like the real lattice, the reciprocal lattice is also a hexagonal network of carbon atoms, which can be expressed as 9

22 (2.15) (2.16) (2.17) where and (2.18) Graphene, because it has a hexagonal structure, has three high-symmetry points. These are labeled as K, M and Γ, as seen in fig. (2.3) Figure 2-3: Hexagonal lattice of graphene with first Brillouin zone and the three high-symmetry points The electronic band structure for graphene was first obtained by Wallace by using the tight binding method [14]. The energy dispersion relation is (2.19) 10

23 Figure 2-4: 3D image of graphene. The energy bands are near the Fermi line. Conduction and valence bands cross at K and K' [15]. Here, the denotes the valence and conductance bands of graphene respectively. As can be seen in figure 2-4, two linear bands cross the at the Dirac points, which lie on the Fermi line, without a band gap. The electronic band structure can be seen in figure 2-5. The Fermi line is located at 0 ev, which is the point that the π and π* bands meet at the K point of the BZ. The π band represents bonding orbitals in the valence band and the π* band represents anti-bonding orbitals in the conduction band. From here we can derive the electronic structure of SWNTs from that of the graphene structure by imposing periodic boundary conditions along the circumference. Along the axis of the nanotube, the wave vector is and along the circumferential direction of the nanotube the wave vector is. The wave vector becomes quantized, however, the 11

24 wave remains the same for an infinite SWNT as it did for graphene, as it is associated with the direction of the translation vector along the tube axis. The wave functions that are allowed for the electrons should satisfy the boundary condition around the circumference, i.e.. The wave function must also satisfy Bloch's theorem, which states that the wave function must be a product of the plane wave and a function, u(r), that has the same periodicity as the lattice. (2.20) (2.21) (2.22) This then leads to the condition (2.23) This makes the wave vector along this direction discrete ( with integer j), but the wave vector perpendicular to remains continuous. These straight lines in k space gives the one-dimensional energy bands in the Brillouin zone. The actual k-value inside the first (1D) Brillouin zone is given by (2.24) We have to include the condition that two wave vectors and are not related to each other by any reciprocal lattice vector in order to make sure that we consider k values to be independent of each other and so that multiple occurrences of the same state aren't counted. The number of bands in the first Brillouin zone can be found by the total number of carbon atoms in a unit cell. This is determined by and T. 12

25 Depending on whether or not the K and K' points in the Brillouin zone of graphene are included in the wave vectors when the graphene is rolled determines the band structure of a nanotube. We see (2.25) where v is an integer (-1, 0, 1) given by the equation (2.26) where N is an integer. Here we see that if then the nanotube is metallic, as the two bands cross at the wave vector (without a gap) corresponding to K and K'. However, when we have a non-zero gap between the conduction and valence bands and thus the nanotube is semiconducting. For another choice of primitive translation vectors and, the integer v is determined by the condition (2.27) where. For a translation of, the Bloch function at the K and K' points gets the phase (2.28) where is determined by (2.29) 13

26 with integer M and is equal to 0 or ±1. The K and K' points are mapped onto and respectively in the one dimensional Brillouin zone of the nanotube. At these points we see that when, two 1D bands cross each other without a gap. In the case where we have a general, we have a chiral structure. Zigzag nanotubes have and armchair nanotubes have. There is only one case where the zigzag nanotube is magnetic, which is when m is divided by 3. Otherwise it is semiconducting. From eq. 2.6, we have the equations, which then gives us. In the 1D Brillouin zone for a metallic zigzag nanotube two conduction and valence bands, having linear dispersion, cross at the Γ point. An armchair nanotube, however, is always metallic because. Again, from eq. 2.6, we have, which then gives us cross at. These have been summarized in table Therefore, the conduction and valence bands always Type Table 2.2: Parameters for zigzag and armchair nanotubes Zigzag 0 0 Armchair 14

27 5.5 ev < 3 ev (a) (b) (c) (d) Figure 2-5: Typical band diagrams for (a) insulators, (b) and (c) metals, and (d) semiconductors As shown in fig. 2.5, in general, insulators have a large energy band gap; the valence band is full and the conduction band is completely empty. Metals on the other hand, can come in two forms, they either have a half-filled valence band or, in the case of metallic SWNTs, overlapping bands (meaning no band gap whatsoever). Semiconductors are similar to insulators, but they have a much smaller band gap, so electrons have a higher probability of jumping to the conduction band. The energy gap of a semiconducting SWNT is inversely proportional to its diameter and can be approximated by the diameter, d, where (2.30) The barrier between a metal and a semiconductor can be identified on an energy band diagram. To build this diagram, we must first look at the band diagrams for a metal and a semiconductor (figure 2-6a and b respectively) and then align them using the Fermi energies. 15

28 E x (a) (b) (c) Figure 2-6: Typical energy band diagram showing (a) a metal and (b) a semiconductor and the Schottky barrier formed once together. The barrier height,, is defined as the potential difference between the Fermi energy of a metal and the band edge, where the majority carriers lie. For an n-type material, the barrier height is calculated as (2.31) where is the work function of the metal and is the electron affinity. The work function and electron affinities of select metals used throughout this work is presented in table 2.3 (Work Functions for Photoelectric Effect). For a p-type material, the barrier height is given by the difference between the valence band edge and the Fermi energy in the metal, seen as (2.32) A metal-semiconductor junction will therefore form a Schottky barrier for electrons and holes if the Fermi energy of the metal as drawn on the diagram is somewhere between the conduction and valence band edge [16]. A Schottky barrier is a non-ohmic contact, i.e. it does not have a linear current-voltage (I-V) curve. 16

29 We define the built in potential of as the difference between the Fermi energy of the metal and that of the semiconductor. (2.33) (2.34) Table 2.3: Work functions of metals used [16] Al Au Cu 4.08 ev 5.1 ev 4.7 ev The density of states for both metallic and semiconducting nanotubes can be derived from the dispersion relation for graphene, i.e. equation (2.19). The dispersion relation close to the Fermi point is (2.35) If we set the parallel and perpendicular components of the k vector as and respectively, then the dispersion relation becomes (2.36) From here, we see that (2.37) The condition for metallicity is, where is a non-zero integer, and the minimum value of, giving us 17

30 (2.38) which is a metallic nanotube with a constant DOS when k is around. Otherwise it is a semiconducting nanotube, which has a band gap. We can calculate the density of state by first need to express the number of states per unit length in a one dimensional system, (2.39) The energy for a free electron wave is expressed as (2.40) And thus the number of states is (2.41) Which gives the density of states as (2.42) The DOS equation in (2.42) is also known as the Van Hove singularity in one dimensional nanostructures. From here we can calculate the density of states for metallic nanotubes the same way, from equation (2.38) (2.43) (2.44) (2.45) 18

31 In the case of semiconducting SWNTs, the distance between the Fermi point and the allowed state is equal to [17] and thus the dispersion relation can be expressed as (2.46) The band gap is non-zero and is seen as (2.47) Again, the density of states can be found, (2.48) (2.49) (2.50) The conservation of momentum and energy laws must apply when dealing with the electronic transition between the conduction and valence band. This electronic transition is given as [17] (2.51) where, is the length of the carbon-carbon bond with a value of roughly nm [18], and is the nearest neighbor carbon-carbon activation energy with a value of approximately 2.75 ev [19]. The value of n varies; it is equal to 1, 2, and 4 for the first, second, and third van Hove transitions, respectively, in semiconducting tubes 19

32 Figure 2-7: Density of states, showing the valence (negative) and conduction (positive) bands, and the Fermi energy (at 0 ev) for a) two zigzag (n,0) nanotubes, the metallic (9,0) and semiconducting (10,0) nanotube, including the DOS for the 2D graphene sheet (in red), and b) four zigzag (semiconducting) nanotubes of different indices. The spikes represent the Van Hove singularities [20]. and it is equal to 3 and 6 for the first and second van Hove transitions, respectively, in metallic nanotubes [18]. Thin films of entangled SWNTs can be both highly transparent and electrically conducting. These thin films are being investigated for use in applications which rely on transparent conductive oxides (TCO), such as. SWNT films have many important benefits over the traditional TCOs; due to the abundance of carbon, the SWNTs potentially have a lower cost. Thin SWNT films are preferentially hole-conducting, highly amenable to low temperature solution deposition and very flexible. However, SWNTs have not yet reached the optoelectronic performance and thermal stability of traditional TCOs. 20

33 The conductivity of SWNT films s dependant on many different things, such as aspect ratio, doping, and network morphology. Due to the difference in behavior of nanotubes (metallic and semiconducting), this should also affect the conductivity. The transport behavior, on the other hand, is more complicated for SWNTs, as synthesized soot is typically a mixture of ⅓ metallic and ⅔ semiconducting nanotubes. Thin films made of soot show a negative temperature dependence of resistance at low temperature and a change to positive dependence above a transition temperature (T*). This U-shaped temperature dependence is well known and has been widely reported and has been attributed to a transition from semiconducting to metallic transport behavior. Many aspects of the electrical transport are still not fully understood; some have reported that the transport is limited by Schottky barriers between semiconducting and metallic SWNTs at low temperature, while at higher temperatures the temperature is thought to be controlled by "percolation pathways" between the metallic SWNTs. This implies that a network of pure metallic SWNTs would have a higher conductivity at low temperature than either films formed by pure semiconductor SWNTs, or those formed by a mixture of the two, and a positive temperature coefficient of resistivity. However, recent observations have shown that the electrical transport is dominated by tube-tube junctions and doping effects for both metallic and semiconducting SWNTs. The unique one-dimensional structure of SWNTs allows for ballistic transport, along the length of a single tube. On the macroscopic scale ( 1 nm), the conductivity is dependent on the material in question. SWNTs are considered to be mesoscopic scale, which is between 1 and 100 nanometers, and so quantum transport theory must be applied to be able to understand electron transport. There are three different characteristic lengths 21

34 employed under quantum theory; the mean free path,, is the average length an electron travels before it is scattered, the Fermi wavelength,, is the debroglie wavelength of an electron at the Fermi level, and the phase-relaxation length,, is the length over which an electron remains as a wave. On the macroscopic scale, the resistivity and conductivity can be calculated using Ohm's law. When the length of the material is greater than the phase-relaxation length, which is then much greater than the mean free path, electrons behave as if they were particles (classically). During diffusive motion, the wave function is localized and so elastic scattering occurs; resistance increases as L becomes large, becoming an insulator. When an electron conducts with no phase or momentum relaxation, ballistic transport occurs. Ballistic conduction combined with the SWNTs high current carrying ability makes SWNTs a better conductor than copper at room temperature. However, when measuring film networks, the performance cannot compete with traditional conductors. When more nanotubes are used, more tubes containing defects made during the synthesis process will appear in the film, which makes the overall conductivity lower than calculated. Data suggests that previous studies reporting U-shaped R(T) behavior with the temperature at which the fundamental conductivity mechanism changes from tunnelinglimited to metallic resistance near to room temperature may have been observing artifacts from the thermal desorption of dopants, i.e. measuring. The value of this is not strongly dependent on the type of tube, and varies only from approximately K, depending on the dopant used. At versus T curve decreased, and at, it was found that the slope of the, the opposite happened. 22

35 At, the temperature dependence of the resistivity was weak, suggesting that a tunneling process might be controlling the conductivity at lower temperatures. A tunneling equation has been used many times before, similar to that in equation (2.52), to model resistivity in SWNT networks at all measured temperatures [21], [22]. (2.52) Here, the linear term was used to represent the metal-like conductivity that was ascribed to the high-temperature behavior. The second term was used to represent the tunneling contribution, which is dominant at lower temperatures. Similar data has been previously fitted without the use of the linear metallic term [21]. Since the metallic contribution term is not evident unless dopant desorption occurs, can be set to zero, thus omitting the term completely and using only the tunneling portion of the equation to fit the data. The tunneling term was originally derived by Sheng, where it was applied to a variety of disordered materials, showing fluctuation-induced tunneling [23]. can be described as the temperature above the point where fluctuation effects become important, however, is a more complex function of the tunneling barrier height and shape as affected by the image force and local electric field. Smaller values of indicate an effectively lower barrier height; it is reduced moreso for the semiconductor-enriched films than for the metal-enriched films, suggesting that the p-type dopants increase conductance through tunnel junctions between semiconducting SWNTs than junctions between metallic SWNTs. is a weak function of temperature that also accounts for the barrier shape and network properties, and it can be considered constant in comparison to the exponential portion of equation 2.52 [23]. 23

36 At, Barnes et al found that the conductivity was to be dominated by fluctuation-assisted tunneling across tube-tube or bundle-bundle barriers [24]. These barriers are the smallest in highly doped, almost pure semiconducting SWNT networks, most likely contributing to the observed higher conductivity in these networks. Data collected has suggested that semiconducting nanostructure networks might be excellent candidates for transparent conducting materials if they can be stably doped. Magnetic nanostructure networks may also prove fruitful, however it has been suggested that finding network materials with small tunneling barriers (low values) is more important for conductivity than just producing a network from the most conductive tubes. Recently, it has been seen that electrical transport in thin SWNT films is dominated by tube-tube junctions and doping effects in both the metallic and semiconducting SWNT enriched films [25], and not, as previously reported, limited by Schottky barriers between the metallic and semiconducting nanotubes at low temperatures, and percolation pathways between metallic SWNTs at higher temperatures [26]. Semiconducting SWNTs have very high mobilities, of the same magnitude as metallic SWNTs, in which holes are localized in the semiconducting SWNT valence band by a strong applied field. Also, it has been noted that junctions between semiconducting SWNTs have comparable transmission probabilities to junctions formed between metallic SWNTs. These observations suggest that degenerately doped semiconducting SWNTs have similar transport parameters to intrinsic metallic SWNTs and should likewise form conductive percolation pathways in SWNT thin films. Essentially, the two should be interchangeable in models where metallic conduction is assumed to dominate the transport behavior in three dimensional SWNT networks. 24

37 As mentioned earlier, SWNT soot naturally consists of approximately ⅓ metallic and ⅔ semiconducting SWNTs. Because of this complex mixture, a detailed understanding of the phenomena that control the overall optical and electrical transport properties of thin, transparent SWNT films has yet to emerge. It is still unclear whether a film with only metallic SWNTs would be a better transparent conductor than a film with only semiconducting SWNTs. While reports exist on the conductivity of single semiconducting or metallic SWNTs [27], [28], single tube transport studies yield idealized transport parameters that might not be applicable to thin films, where the tubetube interfaces may be important. The junctions between nanotubes and/or nanotube bundles may also play a large role in the transport properties of the films. The intersection of two nanotubes, regardless of type, form tunneling barriers for carriers, while junctions between metallic and semiconducting SWNTs form Schottky barriers. Therefore, even if significant density carrier is available for conduction at the single tube level, the properties of tube-tube barriers influence the degree of carrier delocalization over the bulk of the film. Extrinsic factors, such as morphology, non-nanotube (e.g. amorphous) carbon content, and residual surfactant may also affect the optoelectronic properties of the films. Difference in tube length, junction density, and the density of residual amorphous carbon could affect the morphology and electrical transport properties of films. It has been found that when sonication times were longer, morphological differences and different magnitudes of the resistivities were seen [29], but the same trends were always found. So even though changes in sonication can give different length distributions, 25

38 variations in tube length between semiconducting and metallic SWNTs and overall film morphology are quite insignificant. Another factor that could influence the conductivity is the presence of residual surfactant, which could block transport between nanotubes at junctions. Geng et al. recently suggested that the conductivity of nitric-acid-treated SWNT thin films increased by almost three times due to the removal of surfactant [30]. However, the doping-induced changes that occur are all completely reversible. This implies that the large conductivity enhancement induced by the nitric acid soaking cannot be attributed solely to the removal of surfactant. It has therefore been concluded by Blackburn et al. that the main optical and electrical effects from the various chemical treatments may be best evaluated in the context of changes to the intrinsic resistance of semiconducting and metallic SWNTs, as well as the junction resistances [25]. They suggest that interactions associated with tube-tube junctions primarily control the resistivity in these films. The FET mobility in single SWNTs is extremely high, in the order of cm²/vs, resulting in mean free path lengths of the order of 0.5 to several microns, assuming an effective mass of 1 and Fermi velocity of m/s. Considering that this distance is much longer than the typical distance between tube-tube junctions, it is reasonable to conclude that the unexpectedly high resistance of SWNT films is due to the large density of the tube-tube junctions. Each junction creates a tunnel barrier, through which electrons must propagate with some finite transmission probability. Thus, carriers localized on one SWNT can either tunnel into an adjacent SWNT with some probability that depends on tube-tube 26

39 barriers or remain localized on the SWNT. Those carriers that remain localized do not contribute to DC transport, whereas the localized ones do. 2.4 Characterization of Multi-Walled Nanotubes These are made up of many rolls of layers of graphene. There are two different models that can be associated with MWNTs to describe the structure; these are known as the Russian Doll model as in figure 2-3 and the Parchment model. The Russian Doll model says that sheets of graphite are rolled concentrically around each other, i.e. a SWNT inside a larger SWNT, inside a larger SWNT and so on. In the Parchment model, a single sheet of graphite is seen to be rolling in upon itself, similar to a rolled up newspaper. The Russian Doll model is seen more often as its interlayer structure resembles many different SWNTs, which can be either metallic or semiconducting. Figure 2-8: Russian Doll model of MWNTs. All 3 SWNTs in this case are Armchair nanotubes. 27

40 Double walled nanotubes (DWNTs) are a special kind of nanotube because their properties are similar to that of the SWNT but they have a greater resistance to chemicals. This can be especially useful when functionalization (surface modification) is done to add new properties to the carbon nanotube. When covalent functionalization occurs to SWNTs, some of the carbon-carbon double bonds break and holes are therefore formed. This modifies both the mechanical and the electrical properties of the nanotube. However, when functionalization occurs to DWNTs, only the outer wall is modified. The comparison of SWNTs and MWNTs can be seen easily in table 2.4. A single, rolled layer of graphene. A catalyst is required for synthesis. Table 2.4: Comparison of SWNTs and MWNTs [4] SWNT MWNT Bulk synthesis is difficult, as it requires proper control over growth and atmospheric condition. Purity is low in soot. A chance of defect is high during the functionalization process. Less accumulation in the bulk. Characterization and evaluation of nanotubes is easy. It is very flexible, due to its high tensile strength. Multiple rolled layers of graphene. Can be easily produced without the need of a catalyst. Bulk synthesis is easy. Purity is high. A chance of defect is lower, however the nanotube is more difficult to improve once a defect has occurred. More accumulation in the bulk. Characterization is difficult due to their complex structure. Not very flexible, due to its many layers. 28

41 Chapter 3 Synthesis and Characterization of SWNTs 3.1 Introduction The growth and synthesis of SWNTs has a large influence on their structure and therefore their optical and electronic properties. We will discuss three different methods used to synthesize SWNTs; arc discharge (AD), laser vaporization (LV), and chemical vapor deposition (CVD). Synthesized SWNTs have a mixture of different diameters, lengths, chiralities, catalyst particles, and impurities of carbon. There are a few different methods, such as ultra-centrifugation and acid refluxing, to help remove different types of impurities in the nanotubes. There are many different characterization techniques available such as scanning electron microscopy (SEM), tunneling electron microscopy (TEM), x-ray diffraction (XRD), UV- VIS spectroscopy, Raman spectroscopy and four point probe measurements, which can be used to look at the optical, physical and electronic properties of SWNTs [31]. Here we will discuss the different methods of synthesis and purification of SWNTs as well as the different methods of purification. We will also look at the different characterization methods. 29

42 3.2 Growth of SWNTs The mechanism of growth has been debated for a long time, with no clear conclusion. Theories produced by different research groups have been found to be contradictory, and so no single mechanism has been established. However there is a widely-accepted and general growth mechanism available. This is to use the CVD method, with a hydrocarbon source. The hydrocarbon breaks down to form carbon and hydrogen molecules. The hydrogen moves away from the carbon molecules, allowing the carbon to be dissolved into the catalyst used in the synthesis process. Once the temperature for the solubility limit of synthesis has been attained, the carbon precipitates out in the form of a cylinder, with no dangling bonds. The decomposition of the hydrocarbon is an exothermic process and thus releases heat into the area where there is exposed metal, whereas the crystallization of the carbon molecules is an endothermic process and thus absorbs heat from the metal's precipitation area. This thermal gradient is what allows the process to continue. There are two general cases of this. The tip-growth model (Fig. 3.1 (a)) shows that when the interaction between the catalyst and the substrate is weak. the hydrocarbon decomposes on the surface of the metal, the carbon diffuses through the metal and the SWNT precipitates out across the metal bottom, which pushes the particle off the substrate. If the top of the metal remains open for more hydrocarbon decomposition, there is a concentration gradient in the metal, which allows the carbon to diffuse and therefore the nanotube continues to grow upward. Once the metal is fully covered, the catalytic activity stops and thus, the growth stops. 30

43 In the base growth model on the other hand (Fig. 3.1 (b)), the interaction between the catalyst and the substrate is strong. The initial decomposition and diffusion take place in a similar fashion to the tip-growth case, but the nanotube precipitation fails to push the metal upward, so it emerges from the furthest part from the substrate, i.e. the top of the metal. Initially the carbon crystallizes as a metal dome, it then grows upward as a seemingly seamless cylinder. More decomposition occurs on the lower surface of the metal and all diffused carbon diffuses upward, through the cylinder. The SWNT grows upwards with the catalyst particle rooted on the base. Figure 3-1: Growth mechanisms for SWNTs (a) tip-growth method, (b) base-growth method [32]. 31

44 3.3 Synthesis of SWNTs There are many different methods of synthesizing SWNTs, however, they all have a few similar required components such as carbon source, high temperature, and the use of a metal catalyst. Each of the different methods produces nanotubes with different lengths and diameters. We will look at the three most commonly-used synthesization methods; arc discharge, laser vaporization, and chemical vapor deposition Arc discharge - AD This method has been found to be the most common way to synthesize SWNTs as it causes less defects during growth compared to other techniques [33] and was actually the first method in which carbon nanotubes were discovered [2]. A cylindrical chamber is filled with helium and contains two electrodes composing of graphite, the anode and cathode. The former of which is filled with nickel, carbon, and yttrium powders to aid the SWNT production. Both of these electrodes are separated by less than one millimeter and are connected to an external DC power supply with a current of A passed between them. A constant pressure of roughly 500 Torr is maintained in the cylinder throughout the process. SWNTs are deposited onto the cathode when the voltage is applied and the anode is vaporized due to a high temperature (~ 4000 C), which induces plasma between the two electrodes. Even though arc discharged SWNTs are highly crystallized, they are also highly impure; the amount of metal particles and amorphous carbon reaches 60-70% in the product. Much purification needs to be done in order to get pure samples. The mean diameter of SWNTs produced is roughly nm, with lengths of several microns [34]. 32

45 Anode Plasma Cathode Fig 3-2: Schematic diagram of the arc discharge system Laser vaporization - LV This method also involves the vaporization of a carbon blend and a catalyst (such as cobalt and nickel). The graphite and metal catalyst are placed in a tube of Helium at roughly 500 Torr and held at about 1200 C to lift out any and other small fullerenes. A pulsed laser is used to vaporize the target, from this a vapor plume forms, which expands and cools rapidly. As this cools, small carbon molecules and atoms condense and form large clusters. The nanotubes nucleate in the vapor phase, coalesce and condense downstream after leaving the furnace [35]. Here we get a very high purity of SWNTs, however the yield is incredibly small, usually in the order of milligrams. The mean diameter of SWNTs produced is roughly nm. Two disadvantages that both AD and LV have is firstly, they both rely on evaporation of carbon atoms from solid targets at extremely high temperatures and lastly, the nanotubes produced are tangled, making the purification and application processes difficult. Laser SWNTs were used for the nanotubes deposited by the membrane transfer method and were both grown and synthesized in-house. An Nd:YAG pulsed laser working at 1064 nm was used to produce the SWNTs and other different parameters (i.e. pulse rate power density, spot size) are optimized during the synthesis. 33

46 Graphite powder was doped with 0.6 atomic percent of each cobalt and nickel, and pressed into a 2.5 cm diameter die by applying 10 tons of pressure. This was then placed into the quartz tube in a molybdenum foil holder. A furnace was then used to heat the tube to 1200 C and then the pulsed laser was used to vaporize the target. The pressure inside the tube was kept at a constant 500 Torr with a constant flow of 100 sccm of argon using a mass flow controller. The vapor was condensed in the cold region at the top of the quartz tube. After cooling, the nanotube soot was collected and purified Chemical vapor deposition - CVD Unlike the previous two methods, the carbon in this method is supplied as molecules that must be cracked before the growth stage. CVD synthesis is achieved by injected a hydrocarbon (e.g. methane, acetylene) and decomposing it at a relatively high temperature of around 900 C. The carbon gas reacts with the metal catalyst particles and SWNTs are grown on the substrate [36]. We can compare the outcome to the PVD methods, where entangled ropes of SWNTs are the result, to the mostly single (~90%) SWNT results of the CVD method. The diameter of the nanotubes it largely a factor of the diameter of the catalytic particles. This gives us a range of nm [37], with lengths having the range of microns. This means that SWNTs grown using the CVD method are much longer than that produced by the other methods. Unlike the other methods, the SWNTs synthesized by the CVD method can be grown directly on the desired substrate. 34

47 3.3.4 CoMoCat Method In this method, the active cobalt species is kept in a non-metallic state by an interaction with molybdenum oxide before being reduced by the carbonic compound. When it is exposed to carbon monoxide, the Co-Mo dual oxide is carburized to form molybdenum carbide and small metallic cobalt clusters. These clusters remain in a high state of dispersion and result in high selectivity toward SWNT of small diameter. Synthesis at lower temperature and stabilization of metal clusters yields a CoMoCAT nanotube, with a smaller average diameter and a smaller distribution of structures in comparison to other methods outlined. This process uses fluidized bed reactors to keep total control of the temperature and flow rates, which results in high (n,m) selectivity. The nanotube soot collected via this method were enriched with semiconducting nanotubes. Figure 3-3: A fluidized bed reactor. Using the CoMoCAT method, we can scale up the production of SWNTs 35

48 3.4 Purification of SWNTs SWNTs synthesized in any of the above methods contain nanotubes with different parameters, i.e. diameters, chiralities and lengths, as well as many impurities, e.g. graphite sheets, amorphous carbon, and catalyst molecules [38]. Though many different purification techniques exist, a simple method using acid reflux and thermal oxidation has been used. Acid refluxing is a simple method to help remove any metal catalyst particles and amorphous carbon present in the carbon soot sample [39]. Approximately 80 mg of the carbon soot was refluxed for 16 hours in boiling 4M HN. Filtration was then done using a 0.2 mm polytetrafluoroethylene-coated polypropylene filter to collect any solids, this was then rinsed with DI water. The left over carbon mat was removed from the membrane and then heated to about 1100 C for 30 minutes in a environment to remove any impurities, leaving pure SWNTs. Another method used for purification is ultracentrifugation, in which the carbon soot was dispersed in a surfactant using probe sonication. The solution was again ultracentrifuged to get the pure form of SWNTs [40]. Approximately 80 mg of soot was dispersed in 80 ml of 1% SDS-DI water solution using probe sonication at roughly 25 watts with 1/4" tip size for one hour. The solution was centrifuged for an hour with a Ti45 rotor at 45K rev/mins speed and at g. During the centrifugation process, dense metals and other unwanted carbon molecules are collected at the bottom of the tube. The top 80% of the tube was decanted and recentrifuged for a further 4 hours using a SW28 swing motor at 28K rev/min and g to remove any remaining unwanted particles. Again, the top 80% of the tube was decanted and used as the pure SWNT solution. 36

49 3.5 Fabrication of films Before we can apply nanotubes, they must be in the form of thin films. There are many notable techniques that have been used to successfully fabricate these cells. There are two main different methods used for SWNT deposition; direct CVD growth [41], and solution methods [42]. Films grown by CVD generally have tubes with long lengths and the ability to control the alignment of the tube growth in the film [41]. However, because of the high temperature needed to process these, this method cannot be used for samples which are thermally unstable. These are only usually useful in small electronic applications. The films for a larger area application as a transparent electrode need large area depositions, which can be deposited using a solution process. These have more flexibility in processing and controlling the films, but the surfactant still needs to be removed from them. In this work, the SWNT films were fabricated using the solution processes. The following sections describe how the solutions are prepared and how the films are made Preparation of cells A stable solution of SWNTs is needed before the fabrication stage of the films. They need to be dispersed in DI water with the surfactant in order to make a stable suspended solution. The purified nanotubes are dispersed in a surfactant/di water solution using a high powered sonicator. Because the pure SWNTs are in bundles of individual nanotubes tangled together by Van der Waal's forces, high powered sonication is needed to disperse the SWNTs in the solvents. SWNTs themselves are naturally hydrophobic, so they will not naturally disperse in aqueous solutions. 37

50 By dispersing the solution, we are creating a suspension of separated nanotubes, which can be used in future applications. There are two different ways of dispersing the SWNTs, the mechanical method and the chemical method. The mechanical method, for example probe sonication, separates the nanotubes from each other but can easily break the nanotubes. The properties of SWNTs are dependent on their length and bundle size. The chemical method on the other hand uses different surfactants to change the surface energy of the nanotubes and improves their wetting or adhesion characteristics. Using concentrated acids at high temperatures can easily damage the nanotubes. Both of these methods change the size, aspect ratio and distribution of SWNTs in the solution. It has been found that SWNTs are soluble in only a few organic solvents, such as nitromethane, N,N dimethylformamide (DMF), dimethylacetamide (DMAC) and dimethylpyrrolidone (NMP) [43], [44]. In order to disperse SWNTs in the solution, high power sonication is needed. Excessively sonicating the SWNTs can easily damage them and thus electrical degradation is possible [45]. As previously discussed, the SWNTs are non-polar and hydrophobic, so bundles of SWNTs don't break up and disperse very easily into water, however, water with amphiphilic molecules (molecules possessing both a hydrophilic head and hydrophobic tail) such as surfactants can be used to disperse the SWNTs in water. The hydrophilic head mixes with the water, while the hydrophilic tail absorbs on the surface of the SWNTs. Rebundling of the SWNTs is prevented by the electrostatic repulsion from the surfactant. The SWNT breaks from the bundle during the sonication and creates the stabilized solutions of individual SWNTs by attaching them to the surfactant. Some common 38

51 surfactants that are used to disperse the SWNTs are sodium dodecyl sulphate (SDS) [46] sodium dodecylbenzenesulphonate (SDBS) [47], sodium carboxymethyl cellulose (CMC), and polyvinylpyrrolidone (PVP) [42]. Depending on which surfactant is used, the stability differs. The covalent functionalization of SWNTs can also assist in dispersion by creating interactions with the tube surface and the surrounding environment. The solubility of SWNTs in water can be increased by the covalent functionalization of SWNTs with carboxylic acids (COOH), these are preferentially bonded to the highly reactive defect sites due to large local strains [48]. Surface attached carboxylic acid groups lead to the development of a negative electrostatic charge on the SWNTs and lose their non-polar characteristics and become more hydrophilic. The net surface charges also act as a repulsive force between individual nanotubes and prevent them from bundling Deposition techniques There are many different ways we can deposit SWNT solution after preparation to make films, such as spin coating, dip coating and ultrasonic spray technology. Here we will focus on two, the membrane transfer method and drop casting Membrane Transfer Method This method has been found to be the most straightforward and easiest method to get consistently uniform films [46]. A vacuum is used to pump the SWNTs solution through the different membrane filters. When it passes through these filters, SWNTs are collected on the surface of the paper, which forms an interconnected SWNT network 39

52 film. The thickness of the film created can easily be controlled in two different ways, either the volume of the solution, or the concentration of nanotubes in the solution. After the nanotubes are all collected, the paper is rinsed with deionized water, in order to remove any lingering surfactant adsorbed on the surface of the nanotubes. The filter papers are then kept in methanol, to keep them wet and transferable. The easiest, and most common way to transfer the nanotubes is to expose the paper to acetone vapor and rinse with acetone, if required. The experimental setup can be seen in figure 3-4. Figure 3-4: Vacuum filtration and transfer method of SWNTs [49] Drop Casting The produced CoMoCAT nanotubes were made into a polymer wrapped (PFO) solution of SWNTs; a small volume (in this case 500 µl, for 100 nm) of the solution is put onto the back contact electrodes using a pipette and left to air dry for a few minutes. This leaves a uniform thickness of SWNTs as required. 40

53 3.6 Film characterization The different characterization techniques are needed to find the SWNT propertied such as surface morphology, purity, and the optical and the electrical properties. An increasing amount of control on the properties and characteristics of samples is needed for the development and of new production and applications of SWNTs. These characterization tools should give the information of the SWNTs, such as metal content on the samples, amount and type of impurities, crystalline structure (including any bundling), defect density, aspect ratio (length and diameter), functionalization and type of functionalization. Methods such as SEM, TEM, XRD, UV-Vis-NIR spectroscopy, Raman spectroscopy and the four point probe method have been commonly used. Here we will explain these X-Ray Diffraction X-Ray Diffraction is a non-destructive characterization process, which allows us to determine the purity of SWNTs. Figure 3-5 shows the XRD pattern of soot and purified SWNTs. We see the intense graphite peaks at 2θ = 26.5 and bundle peaks of SWNTs at 2θ = 6, 10, 16, and 20. These are produced by diffraction planes, which are at a distance equal to approximately the diameter of the nanotubes. Similarly, the peak of graphene corresponds to the distance between the graphene sheets. If they are well dispersed, SWNTs act as if they are an amorphous material. Hence, patterns from the XRD describe the nanotube packing and the amount of bundling in samples. 41

54 Figure 3-5: X-ray diffraction of laser produced SWNT soot, purified SWNTs heated to 950 C and 1100 C in a carbon dioxide environment [49] Electron and Atomic Force Microscopy Considering that XRD can be used to look at the purity of SWNT samples, the microscopy techniques discussed in this section can be used to investigate the purity as well as other properties, such as surface and shape, bundle size, and length of SWNTs. TEM and SEM are traditionally the most important techniques used to characterize SWNTs. As previously mentioned, SWNTs were discovered by the use of TEM in 1991 [2], and high resolution TEM is still a valuable tool used to look at nucleation and growth of SWNTs [50]. Both individual and bundle SWNTs can be observed and measured by 42

55 a) d) b) e) c) f) Figure 3-6: a) TEM image of raw SWNT soot from laser ablation, b) SEM image of purified SWNT, c) SEM image of impure SWNT films, d) AFM image of the SWNT films sprayed, e) SEM image of a SWNT bundle in a polymer-wrapped SWNT film, and f) SEM image of polymer-wrapped SWNT film [49] 43

56 using both SEM and TEM. These can also be used to find the residue of metal particles in SWNTs. Figure 3-6a shows typical images of SWNT soot under TEM, in which metal particles can be seen as dark spots. SEM can be used to find purity of SWNTs, such as amorphous carbon, but the quantity of the impurities is not possible. These methods can, however, help us estimate the length and diameter, as well as the bundle size of the SWNTs; the downside to this is that it is very time consuming and difficult, as SWNT bundles are randomly oriented and form an extremely dense and entangled network. Figures 3-6b and 3-6c both show the SEM images of the surface morphology of SWNT films and figure 3-6c also shows a typical SEM image of SWNT film having large bundles and impurities on it. In SEM images, metal particles, amorphous carbon, and graphite are all easily distinguishable from SWNTs, and thus a qualitative assessment of the purity is straightforward. The image area in SEM is larger than that used for TEM, which gives a clear advantage when looking at the purity. The AFM image of SWNT thin films, as shown in figure 3-6d, determines the surface morphology and surface roughness of SWNTs. One major problem with using SEM and TEM is the scale of analysis. The amount of material that is probable in the in a usual SEM frame is less than practically impossible to homogenize SWNTs in order to make a g [38]. It is g sample representative of a 10 g sample. Even by increasing the level, it becomes impossible to achieve a reading with quantitative and significant data in a realistic amount of time [51]. 44

57 3.6.3 Ultraviolet to Near Infrared Spectroscopy Ultraviolet to near infrared spectroscopy is a useful method for the optical characterization of SWNTs. It has been used to find interband transitions [19], [52], evaluate the effect of ionic and covalent chemistry on the band structure [52], monitor the efficiency of purifications [53]. This can be done as both a solution and as a thin film. The spectra shown in figure 3-7(a) shows the absorption of the LV produced SWNT film. We can see that the peaks correspond to the semiconducting nanotubes and the peak corresponds to the metallic nanotubes. The spectra in figure 3-7(b) shows us the absorption of CoMoCAT SWNTs for both the solution and film. Kataura et al discovered the optical properties of SWNTs, such as the relation between excitation energy and diameter, chirality, and characteristic of the SWNTs [19]. The positions of the peaks are important in telling us the band gap of the nanotubes, which differs depending on how they are synthesized, as seen in figure 3-7. UV-Vis data can also be used to determine other important information about the composition of the sample; Itkis et al showed that the purity of SWNTs can be found by utilizing the region of the second interband transition for semiconducting SWNTs [38], and Blackburn et al showed that the ratio of metallic to semiconducting SWNTs are easily visible [25]. 45

58 Absorption (a.u.) S S Wavelength (nm) Figure 3-7: UV-Vis spectroscopy of (a) LV SWNTs and (b) CoMoCAT SWNTs [49] showing the metallic and semiconducting peaks 46

59 3.6.4 Raman Spectroscopy Raman spectroscopy is a commonly used technique for the characterization of SWNTs. The Raman features of the SWNTs depend heavily on the individual nanotube characteristics, such as quality and diameter of the nanotube. The spectra shown gives us useful information about the diameter, chiralities, electronic properties, crystallinities and degree of functionalization of the SWNTs. The scattering from SWNTs is resonantly enhanced if the excitation energy matches the separation between the Van Hove singularity pairs in the one dimensional electronic density of states of the SWNTs. The two most obvious features observed in the first order resonant spectrum are the radial breathing mode (RBM) at the low-frequency ( ) end, which is made by carbon atoms vibrating in phase in the radial direction of the SWNTs; and the tangential (G) mode at the higher frequency ( ) end, which is made up of several tangential modes due to stretching vibrations of the SWNT carbon-carbon bonds in the axial direction [20], [54]. The RBM frequency can be used to determine the diameter of the SWNTs in the sample and the G- mode can be used to differentiate between the semiconducting and metallic nanotubes [20], [54]. Also seen in figure 3-8 is the low-intensity D-band, which is associated with any disordered sp³-hybridized carbon present as defects and impurities in the SWNT sample [20], [54]. The ratio between the amplitudes of this band and the G-mode can be used to measure the purity of the sample and thus the efficiency of the purification. The spectral width of the D-band has also been seen to be a measure of the purity [39]. 47

60 Figure 3-8: Raman Spectroscopy of LV and CoMoCAT SWNT films on glass excited with wavelengths of 633 nm [49] Another present mode, known as the G'-mode, is seen after the G-mode, at approximately 2600 and is due to the second order harmonic of the D-band. Analysing this allows us to determine the indices of the SWNTs, and therefore whether they are semiconducting or metallic in nature Four Point Probe Method It has been predicted that SWNTs have amazing ballistic transport properties at the single nanoscale level. The resistivity and conductivity measurements of SWNTs are usually done by using the four point probe technique. This method is an electrical impedance measuring technique, which uses separate pairs of voltage and current 48

61 electrodes as shown in figure 3-9, and is more accurate than the, much simpler and more common, 2 probe method. Electrical and thermal conductivity of SWNTs depend entirely on the purity and quality of grown SWNTs. When used in optoelectronic devices, the conductivity is an integral component of the resistive power losses in optical and other devices. The power loss can be seen as (3.1) where I is the current through the electrodes and is the sheet resistance of the film, used to characterize the 2 dimensional electric properties of the semiconductor. A V Figure 3-9: Collinear arrangement of the four point probe Current is supplied to the two outermost wires, which in turn produces a voltage drop across the impedance, which is measured by using Ohm's Law, (3.2) A pair of voltage connections are made immediately next to the target impedance, so that they do not include the voltage drop in the current leads or contacts. Since almost no current flows to the measuring instrument, the voltage drop in the sense leads is negligible. Assuming that the spacing between the probes are equal, then the resistivity is given by 49

62 (3.3) Where s is the spacing between the probes. This measured resistivity is only equal to the actual value if the sample is of semi-infinite volume. This is not the case in real world situations, where the sample is of finite size. As found by Valdes, if the distance from any probe to the edge of the sample exceeds 5s, then no correction factors are needed [55]. However, if the sample thickness is 5s, we can compute the true resistivity from (3.3) Where a is the thickness correction factor. This value has been gathered from fig 2.6. The value of this is (3.4) so by substitution, the original equation simply becomes (3.5) From here, we can simply divide both sides of the equation by the thickness to find the sheet resistance, the measure of uniformly thick thin films, as shown by (3.6) For a thin rectangular slice, which is defined when thickness, t, is much less than half of the spacing between probes, s, the resistivity can be given by (3.7) 50

63 where is the geometric factor for an infinitely large slice where and is the correction factor we need to apply because of its finite rectangular shape. Figure 3-10: Finding the thickness correction factor, a, for different thickness-spacing ratios 51

64 Table 3.1: Values for the correction factor,

65 Current (ua) K 86.25K 157.5K K 300K Voltage (V) Figure 3-11: Four point probe measurements for the polymer wrapped SWNTs. From figure 3-11, and using equation 3.6, the sheet resistance could easily be calculated. The values of this for each temperature measured can be seen below, in table 3.2, as well as the resistivities and conductivities. Table 3.2: Sheet resistances, resistivities, and conductivities for each of the temperatures measured for CoMoCAT produced SWNTs. Temperature (K) Sheet Resistance ( / ) Resistivity ( m) Conductivity (S/m)

66 3.7 Applications of SWNTs SWNTs have been found to have unique properties, including having a thermal conductivity higher than that of diamond, a mechanical strength greater than steel, being lighter than any metal, and having a better electrical contact than copper, meaning SWNTs have become a wide area of interest for many different types of industry. Many current uses and applications have mostly been limited to the use of bulk SWNTs, though these may never reach the same tensile stress as individual SWNTs, they can reach strengths that can be sufficient for many applications. Electronics has recently been a large focus for the future of nanotubes. Possible applications are solution-processed solar cells, field effect transistors, touch screens, and EMI shielding. SWNT films are attractive replacements for traditional transparent conductors, such as indium tin oxide (ITO) in low-cost, flexible, and solution-processed applications due to the natural abundance of carbon, amenability to spraying and printing, and good wetting properties. However, the efficiencies of ITO and ZnO-based cells are not easily surpassed. To produce comparable cells, SWNT thin films need to reach or exceed the low sheet resistance values, approximately 5 to 10 /sq of high quality ITO at comparable transparencies, about 85% across the visible spectrum. Thin SWNT films are being investigated for use in applications which rely on transparent conductive oxides (TCO), such as. Much attention has been on the application of SWNTs in nano-electronics, with the theory that nanotubes can be useful in downsizing circuitry due to the one dimensional nanostructure. The surface structure minimizes issues related to surface states and 54

67 roughness, which are abundant in conventional semiconductor technology. The smooth surface of the SWNT, and thus the lack of dangling bonds, decreases potential scattering and therefore increases carrier mobility. SWNT thin films are also potentially useful in field effect transmitters (FETs) and as transparent conductors. -mention papers- Different forms of carbon are already in use in fuel cells and other electrochemical devices and have been used for a long time. The electron transfer kinetics are faster on nanotubes than any of the other forms; this depends on various factors, such as morphology and structure of the carbon used, and determines the efficiency of the fuel cells. SWNT micro electrodes are used in bioelectrochemical reactions, which has been found to be superior to other carbon electrodes, in terms of reaction rates and reversibility. Nanotubes could potentially be excellent replacements for conventional carbon-based electrode cells. Electronic devices have also been looked at, focusing on the application of SWNTs in the field emission electron sources for different technologies, such as flat panel displays, gas discharge tubes, and microwave generators. Due to the smaller radius and larger length of SWNTs, high fields can be applied between the SWNT electrode and other anode electrodes. SWNTs have recently been use in biomedical applications, such as drug delivery, selective imaging and others [4]. The toxicity of SWNTs have been thoroughly investigated, however, the results have been so far inconsistent. Despite this, it is agreed that well-dispersed SWNTs have little-to-no toxicity and are therefore safe to use in these applications. 55

68 For over a decade, research has been increasing on using SWNTs in solar cells and they have also been considered for use in energy conversion and storage. SWNTs have been interfaced with both inorganic and organic materials to construct devices for light energy conversion. Fujiwara et al. observed the photoconductivity of films, having two peaks in the excitation spectra at approximately 0.7 ev and 1.2 ev at room temperature; these have been interpreted as photocurrent in semiconducting SWNTs. Semiconducting SWNTs have high absorbance in the visible and near infrared spectrum, with a tunable bandgap depending on their diameter. SWNTs also have a very high charge mobility for both individual nanotubes and SWNT films, in comparison to conductive organic materials. The use of SWNTs is preferred because they have high stability, are easily fabricated, cost effective as they can be easily incorporated into the device, and can be easily doped. Armchair nanotubes have recently been found to help improve the efficiency of the power grid in the USA ("TheEngineer", 2011). Scientists in Texas found that replacing the existing copper-based wires with their armchair quantum wires could reduce the amount of the electricity leaked, which is currently estimated at about five per cent per one hundred miles. Armchair nanotubes have been found to be the only type that is metallic. All others, i.e. Zigzag and Chiral, are semiconducting. 56

69 Chapter 4 Low Temperature Measurements on SWNTs 4.1 Introduction In this chapter we will discuss the process of using a cryostat to procure low temperature measurements, and how exactly they work. We will also go into the four different types of cryostat available, including the differences between them. The closedcycle cryostat will be focused on the most, as that was the setup used to take the samples to low temperatures and then take measurements. 4.2 Cryostats A cryostat is a device which is used to maintain a low temperature of samples mounted within the device. There are four main types of cryostat; closed-cycle, continuous flow, bath, and multistage. These all have the potential to go down lower than 10 K. 57

70 4.2.1 Closed-Cycle Cryostats These have a chamber which has cold helium vapor passing through. The warmer helium vapor is extracted by an external refrigerator, cooled, and then cycled back into the refrigeration cryostat. These have been found to consume a lot of electrical power, but the helium never needs to be refilled and can run for an indefinite period of time. The cryostat in use for this experiment is cryogen free. Cryogen use has been associated with several health risks and operational problems [56]. It has a welded stainless-steel construction to allow for high-vacuum Components of the Closed-Cycle ARS DE Cryos1tat Figure 4-1: Diagram of set up and components of the ARS DE closed-cycle cryostat [57] 58

71 The expander (aka cold head, cold finger) is where the Gifford-McMahon refrigeration cycle takes place. It is connected to the helium compressor by two-way gas lines and an electrical cable. One of these gas lines supplies high pressure (low temperature) helium gas to the expander and the other returns low pressure (high temperature) helium gas from the expander. The compressor allows the necessary flow rate of helium gas at high and low pressure for the expander to convert into the desires refrigeration capacity. The vacuum shroud surrounds the cold end of the expander in a vacuum, which limits the heat load on the expander, caused by conduction and convection. The radiation shield is cooled by the first stage of the expander and insulates the second stage from the room temperature thermal radiation being emitted from the vacuum shroud. A Lakeshore 335 temperature controller was used to measure and adjust the sample temperature. Cooling water is fed into the compressor in order for the helium to remain cold The Gifford-McMahon Refrigeration Cycle The closed-system cryocooler system operates on a pneumatically driven Gifford- McMahon cycle (GM cycle). This is different to a mechanically driven cooler in that it uses an internal pressure differential to move the displacer instead of a mechanical piston, resulting in smaller vibrations. The refrigeration cycle of the closed-cycle cryostat starts with the rotation of the valve disk, opening the high-pressure path, allowing the high pressure helium gas to pass through the regenerating material into the expansion space. Second, the pressure 59

72 Figure 4-2: The GM refrigeration cycle (a) block diagram of the internal structure of an ARS closed cycle cryocooler expander; (b) steps 1 & 2 of the GM cycle & (c) steps 3 & 4 of the GM cycle differential drives the piston up, allowing the gas at the bottom to expand and cool. Third, the rotation of the valve disk opens the low pressure path, allowing the cold gas to flow through the regenerating material, removing heat from the system. Finally, the pressure differential returns the displacer to its original position, completing the cycle Measurements Once the sample was placed in the holder and all pins were attached and giving stable readings, the radiation shield and shroud were placed on the cryostat, carefully to ensure the pins didn't move. The vacuum pump was then connected and once the system was pumped down to 1.8 millitorr, the compressor was switched on. Readings were taken in increments, from 300K down to 15K. Once 200K was reached, the valve 60

73 connecting the cryostat and the vacuum was closed, to ensure that there wasn't any low pressure gas return Continuous-Flow Cryostats These are cooled by liquid cryogens from a storage Dewar. As the cryogen boils inside the cryostat, it is continually replenished by a continuous flow from the Dewar. Temperature control of the sample within the cryostat is done by controlling the flow rate of the cryogen, together with a heating wire connected to a PID temperature control loop. Unlike the closed-cycle cryostats, the length of time which cooling can occur depends entirely on how much cryogen is available. Some laboratories have created systems to capture and recover the helium as it escapes from the cryostat, though these facilities are extremely expensive to run Bath Cryostats These are similar in construction to vacuum flasks, an insulating storage vessel that lengthens the time over which its contents remain colder (or hotter) than its surroundings. The vacuum flask consists of two flasks, one inside the other and joined together at the neck. The gap between these two flasks is partially evacuated of air, creating a vacuum, preventing heat transfer by conduction or convection. In the case of bath cryostats, they are filled with liquid helium. A coldplate is placed in thermal contact with the bath. The liquid helium can be replenished as it boils away, at intervals between a few hours and several months, depending on the volume and construction of the cryostat. The boil-off rate is minimized by shielding the bath with either cold helium 61

74 vapor or vacuum shield with walls made of a super insulator material. The helium vapor, which boils away from the bath very effectively, cools thermal shields around the outside of the bath. Older designs had additional liquid nitrogen baths, or several layers of shielding with gradually increasing temperatures. However, the invention and use of super-insulator materials has made this practice obsolete Multistage Cryostats To achieve temperatures lower than liquid helium, additional cooling stages may be added to the cryostat. Temperatures as low as 1 K can be reached by attaching the cold plate to a 1-K-pot, which is a container of He-4 isotope which is connected to a vacuum pump. Temperatures as low as 1 mk can be reached by using a dilution refrigerator or dry dilution refrigerator in addition to the main stage and 1-K-pot. Temperatures even further below this can be reached using magnetic refrigeration, which is based on the magnetocaloric effect. 4.3 Data acquisition Data was acquired using a method similar to that of the four point probe method; as seen in diagram 4.5, connections were made such that the current and voltage input were the same contact and the current an voltage output were made the same contact, so we had a two pin contact in total. The back contact of the sample (i.e. the electrode on the bottom) was connected using the pin at the back of the sample holder, and thus the front contact was connected using the pin at the front of the sample holder. 62

75 Current (ma) Current (ma) Current (ma) Current (ma) Current (ma) Current (ma) K 86.25K 157.5K K 300K Voltage (V) Voltage (V) Voltage (V) Voltage (V) Voltage (V) Voltage (V) Figure 4-3: Comparison of the two different SWNT samples used, for all three symmetrical electrodes; (a) Aluminum, (b) Gold, and (c) Copper with PFO CoMoCAT SWNTs, and (d) Aluminum, (e) Gold, and (f) Copper with Laser Vaporized SWNTs. From the graphs shown in figure 4-3, it's clear to see that as temperature decreases, conductivity increases. Calculating the differentiation on these curves yielded a symmetrical curve for all, meaning that each of these three were perfectly symmetrical. For the data in figure 4-4, different metallic electrodes were used, and their accompanying asymmetries can be seen. The change in conductivity is the same, regardless of the electrode or type of SWNT used between the contacts. 63

76 Current (ma) di/dv (x10-3 ) Current (ma) di/dv (x10-3 ) Current (ma) di/dv (x10-3 ) K 86.25K 157.5K K 300K Voltage (V) Voltage (V) Voltage (V) Voltage(V) Voltage (V) Voltage (V) Figure 4-4: Three of the asymmetrical contacts, (a) Gold-Copper, (b) Aluminum-Copper, and (c) Copper-Gold, as front-back contacts respectively, with polymer-wrapped CoMoCAT SWNTs. From both figure 4-3, we can see that the contact between these metals and the SWNTs is non-ohmic at temperatures lower than room temperature (300 K), i.e. that there is not a linear plot, and thus there must be a Schottky barrier present. However, figure 4-4 shows that the contact between the asymmetric metal electrodes and the SWNT is non-ohmic for all temperatures. This asymmetry in the I-V measurements occurs because of the differing heights of the Schottky barriers under forward and reverse voltage bias. We can calculate the height of this Schottky barrier, first by calculating the resistance and using 64

77 Current (ma) this to find the resistivity and conductivity, and then finally the activation energy required to overcome the barrier height can be seen in equations As seen in figure 4-5, the Al and Au samples both give a perfect exponential curve for both positive currents (forward bias) and negative currents (reverse bias) Al-SWNT-Al Cu-SWNT-Cu Au-SWNT-Au /T (K^-1) 50 60x10-3 Figure 4-5: Symmetrical exponential curves of current versus the inverse of temperature for aluminum, copper and gold laser SWNTs at ±1.15 V 65

78 Current (ma) 2 1 Al-PFO-SWNT-Al Cu-PFO-SWNT-Cu Au-PFO-SWNT-Au /T (K^-1) 50 60x10-3 Figure 4-6: Symmetrical exponential curves of current versus the inverse of temperature for aluminum, copper and gold CoMoCAT SWNTs at ±1.15 V Figure 4-6 shows 1/T versus Current for a chosen voltage of ±1.15 V for the CoMoCAT SWNTs. A similar pattern to figure 4-5 can be seen, where the Al and Cu samples both give a perfect exponential curve for both forward and reverse bias. The curves in figure 4-7 show the asymmetric electrodes and their effect, here we see that none of the points lie on the exponential curve. 66

79 Current (ma) 4 Al-PFO-SWNT-Au Al-PFO-SWNT-Cu Au-PFO-SWNT-Al Au-PFO-SWNT-Cu Cu-PFO-SWNT-Al Cu-PFO-SWNT-Au /T (K^-1) 50 60x10-3 Figure 4-7: Symmetrical exponential curves of current versus the inverse of temperature for the six asymmetrical electrodes with CoMoCAT SWNTs at ±1.15 V Resistivity can be calculated by the equation (4.1) where R is the resistance, calculated by ohm's law (equation 3.1), L is thickness of the film, and A is the surface area. The conductivity can be easily calculated as the inverse of the resistivity, i.e. (4.2) By plotting the conductivity against the inverse of temperature, we get a graph with a similar pattern, as seen in figures 4-6 and

80 Conductivity (S/m) Conductivity (S/m) 6x Al-SWNT-Al Cu-SWNT-Cu Au-SWNT-Au /T (K^-1) 50 60x x Al-PFOSWNT-Al Cu-PFOSWNT-Cu Au-PFOSWNT-Au /T (K^-1) 50 60x10-3 Figure 4-8 Exponential curves of conductivity versus the inverse of temperature for the three symmetrical electrodes with a) laser SWNTs and b) CoMoCAT SWNTs at ±1.15 V 68

81 From here, we can see that energy can be calculated using the Arrhenius relation where is the Boltzmann constant and is the activation energy. Therefore, we see (4.3) (4.4) and thus we can plot versus to calculate the activation energy required to overcome the Schottky barrier height from the slope of the line, as seen in figure 4-9. The graph for the LV SWNTs (figure 4-9a) shows a dual linear curve for all three of the symmetrical electrodes. This shows that the barrier height is constant at high temperature as we expect, and changes as we get to a lower temperature. The same thing happens when plotting for the CoMoCAT SWNTs (figure 4-9b). Values of the activation energy at high temperatures for each of the symmetrical metal contacts can be seen below, in table 4.1. Table 4.1: Schottky barrier height in both laser and CoMoCAT produced SWNTs for five temperatures measured. As the temperature increases, the barrier height also increases. Activation Energy Work Function Metal Contact Laser SWNTs PFO SWNTs (ev) at high Temp. (ev) Al-Al Cu-Cu Au-Au Al-Al Cu-Cu Au-Au

82 ln(conductivity) ln(conductivity) Al-SWNT-Al Au-SWNT-Au Cu-SWNT-Cu /T (K^-1) 50 60x /T (K^-1) 50 60x10-3 Figure 4-9: Dual linear curves of ln( ) versus 1/T for a) laser SWNTs and b) CoMoCAT SWNTs, where the slope is a function of the activation energy 70

83 Chapter 5 Conclusion and Future Discussion 5.1 Summary The use of SWNT films as a tunneling contact has been the main focus of this research. Laser vaporized films were purified and were fabricated using membrane transfer, and CoMoCAT made films were fabricated using the drop-cast method. These were applied to the same metal electrodes. I-V measurements were taken using a Keithley 2401 Sourcemeter and temperature was altered using a closed-cycle cryostat and measured with a Lakeshore 335 Temperature Controller. A nanotube thickness of 100 nm was chosen and the electrode contacts created a total nanotube surface area of 9 mm². Knowing this, the conductivities were calculated for each of the electrode samples and graphed with, showing an exponential decrease. Using equation 4.4, we found the energy required to overcome the Schottky barrier for the SWNT-metal junction to be much higher in the CoMoCAT SWNTs than in the laser produced SWNTs. 5.2 Future Work We see from figure 4-9 at there is a dual linear plot for the activation energy value, from here we can deduce that there is either a point in which the activation energy 71

84 changes at low temperature ( somewhere between 15 K and K) or there is an error in the hardware setup. The sample is was placed half an inch from the thermistor, meaning that the exact temperature was not being read. This could have potentially made the 15 K temperature read more like 30 K, which would have made the slope more linear overall. In the future, we can move the thermistor closer to the sample to get a more accurate reading and also measure more low temperatures between 15 and 100 K to see if there is, in fact, a point at which the activation energy changes. Future work has been proposed on looking at how different light sources, such as a chopped laser and broadband light, affect the height of the Schottky barrier. All measurements done in this body of work were done exclusively in the dark. This will be done on the CdTe solar cells mentioned in chapter 6, as well as the electrode samples. 72

85 Chapter 6 CdTe Solar cells 6.1 Introduction Another thing that was done throughout the work of this thesis, was measurements on CdTe thin film solar cells. These are currently in development as a lowcost replacement to silicon based solar cells. An efficiency of over 19% has been reached in small area devices [58] and over 16% at the module levels [59]. CdTe itself has a bandgap of 1.45 ev and a high absorption coefficient within the solar spectrum. Generally a film with only one micron thick is needed to convert sunlight into energy, whereas c-si solar cells need to be hundreds of microns thick. CdTe solar cells can be fabricated in both substrate and superstrate configurations. In this work, however, a superstrate configuration was used, as seen in figure 6-1. This configuration is composed of a window layer, absorber layer, and metal back contact. It was thought that the SWNTs were acting as a tunnel between the p-type (CdTe) and n-type (Tec15) materials. The comparison of J-V curves between these two cells can be seen in figure

86 CuAu CdTe Tec15 Glass In CuAu CdTe SWNT Tec15 Glass In Figure 6-1: Superstrate configuration of CdTe solar cell (a) without SWNTs and (b) with SWNTs In the cell looked at in this work, TEC15 was used as the window layer and is also a transparent conductive oxide (TCO), this was prepared by Pilkington and coated onto soda lime glass. A strip of indium was used as the back contact and a layer of coppergold was used as the front contact. A laser scribe was used to produce the different cells on the sample. 6.2 Results and Discussion Indium was used as the back contact and CuAu as the front contact. As we can see from figure 6-1, the presence of SWNTs as a medium between the p-n junction makes a big difference to the overall conductivity of the sample. At 300K, the sample without any SWNT has a conductivity ranging from 1.6E-8 to 3.0E-9 and for the sample with SWNT the conductivity ranges from 3.1E-7 to 2.7E-7 within the same voltage range of ±0.5 V. This means that the addition of the SWNT will increase the overall conductivity of the sample and thus the barrier height will also increase, from equation

87 Current (ua) Current (ua) K 41K 72.1K 103K 134K 150K 165K 181K 196K 212K 227K 243K 258K 274K 290K 305K -0.5 No SWNTs Voltage (V) K 86.25K 157.5K K 300K Voltage (V) Figure 6-2: Comparison between CdTe solar cells (a) without and (b) with SWNTs. Because the conductivity of the sample increases with the addition of the SWNTs, we can conclude that the SWNTs act as a barrier between the p-n junction and increase the conductivity of the sample. 75

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93 Appendix A LabVIEW Program Here we have the program made to collect data from the Lakeshore 335 Temperature controller and the Keithley 2401 Sourcemeter through LabVIEW. Figure A-1: The front panel of the program. In figure A.1 the temperature range is inputted, depending on whether a calculation is or a manual input is done. A voltage start and stop is chosen and the number of voltage 81

94 points to be set is also inputted. A settle time is chosen while a temperature equilibrium is reached. A current range can be chosen, or left on auto mode. A J-V graph is displayed immediately after the measurement is taken. Figure A-2: The first section of the program - file name is chosen and the temperature array is calculated and sorted. 82

95 Figure A-3: The Set Temperature sub-vi. Depending on the user's choice, either the step size or number of steps is inputted and the other is calculated and placed in the array on the front panel. 83

96 Figure A-4: The second half of the program. Here the temperature of probe A is read and displayed on the front panel. 84

97 Figure A-5: Here the temperature of probe A is checked to make sure it is within 1% of the set value, if it is, then it moves to the settling stage. If not, the program moves back to "Read A" and checks again. 85

98 Figure A-6: Settling time, waiting for the temperature to equilibrate after reaching 1% of the set temperature. Five minutes was set, as it reached within.01 K. 86

99 Figure A-7: Record section of the program. The Sub VI provides the voltage and current values from the current sweep within the set voltage range, set on the front panel. 87

100 Figure A-8: The sub VI for the J-V measurements. Here the program communicates with the Keithley Sourcemeter 4601, supplying a voltage and measuring the current received. 88

101 Figure A-9: Tabulation of all collected data and labeling with headers, corresponding to the temperature at which the measurement is taken. 89